> install.packages("devtools")
also installing the dependencies ‘gert’, ‘usethis’

Warning in install.packages :
  unable to access index for repository https://cran.rstudio.com/bin/macosx/contrib/4.1:
  cannot open URL 'https://cran.rstudio.com/bin/macosx/contrib/4.1/PACKAGES'
Package which is only available in source form, and may need compilation of
  C/C++/Fortran: ‘gert’
Do you want to attempt to install these from sources? (Yes/no/cancel) yes
installing the source packages ‘gert’, ‘usethis’, ‘devtools’

trying URL 'https://cran.rstudio.com/src/contrib/gert_1.1.0.tar.gz'
Content type 'application/x-gzip' length 67224 bytes (65 KB)
==================================================
downloaded 65 KB

trying URL 'https://cran.rstudio.com/src/contrib/usethis_2.0.1.tar.gz'
Content type 'application/x-gzip' length 306090 bytes (298 KB)
==================================================
downloaded 298 KB

trying URL 'https://cran.rstudio.com/src/contrib/devtools_2.3.2.tar.gz'
Content type 'application/x-gzip' length 373387 bytes (364 KB)
==================================================
downloaded 364 KB

* installing *source* package ‘gert’ ...
** package ‘gert’ successfully unpacked and MD5 sums checked
** using staged installation
Found pkg-config cflags and libs!
Using PKG_CFLAGS=-I/usr/local/Cellar/libgit2/1.1.0/include
Using PKG_LIBS=-L/usr/local/Cellar/libgit2/1.1.0/lib -lgit2
Configuration OK!
** libs
rm -f gert.so branch.o clone.o commit.o config.o conflicts.o files.o init.o merge.o rebase.o stash.o submodules.o tag.o utils.o version.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c branch.c -o branch.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c clone.c -o clone.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c commit.c -o commit.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c config.c -o config.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c conflicts.c -o conflicts.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c files.c -o files.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c init.c -o init.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c merge.c -o merge.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c rebase.c -o rebase.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c stash.c -o stash.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c submodules.c -o submodules.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c tag.c -o tag.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c utils.c -o utils.o
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I/usr/local/Cellar/libgit2/1.1.0/include -DR_NO_REMAP -DSTRICT_R_HEADERS  -I/usr/local/include   -fPIC  -Wall -g -O2  -c version.c -o version.o
clang -mmacosx-version-min=10.13 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o gert.so branch.o clone.o commit.o config.o conflicts.o files.o init.o merge.o rebase.o stash.o submodules.o tag.o utils.o version.o -L/usr/local/Cellar/libgit2/1.1.0/lib -lgit2 -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
ld: warning: text-based stub file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation.tbd and library file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation are out of sync. Falling back to library file for linking.
installing to /Library/Frameworks/R.framework/Versions/4.1/Resources/library/00LOCK-gert/00new/gert/libs
** R
** inst
** byte-compile and prepare package for lazy loading
Error in dyn.load(file, DLLpath = DLLpath, ...) : 
  unable to load shared object '/Library/Frameworks/R.framework/Versions/4.1/Resources/library/openssl/libs/openssl.so':
  dlopen(/Library/Frameworks/R.framework/Versions/4.1/Resources/library/openssl/libs/openssl.so, 6): Library not loaded: /usr/local/opt/openssl/lib/libssl.1.0.0.dylib
  Referenced from: /Library/Frameworks/R.framework/Versions/4.1/Resources/library/openssl/libs/openssl.so
  Reason: image not found
Calls: <Anonymous> ... asNamespace -> loadNamespace -> library.dynam -> dyn.load
Execution halted
ERROR: lazy loading failed for package ‘gert’
* removing ‘/Library/Frameworks/R.framework/Versions/4.1/Resources/library/gert’
Warning in install.packages :
  installation of package ‘gert’ had non-zero exit status
ERROR: dependency ‘gert’ is not available for package ‘usethis’
* removing ‘/Library/Frameworks/R.framework/Versions/4.1/Resources/library/usethis’
Warning in install.packages :
  installation of package ‘usethis’ had non-zero exit status
ERROR: dependency ‘usethis’ is not available for package ‘devtools’
* removing ‘/Library/Frameworks/R.framework/Versions/4.1/Resources/library/devtools’
Warning in install.packages :
  installation of package ‘devtools’ had non-zero exit status

The downloaded source packages are in
	‘/private/var/folders/hd/yz81jsl14dx__mdn2gmctk6c0000gp/T/RtmpQW5saW/downloaded_packages’
Warning message:
R graphics engine version 14 is not supported by this version of RStudio. The Plots tab will be disabled until a newer version of RStudio is installed.