Not able to install umx packaging in R 3.6.3
I was getting the same error for the last 2 days and then I wrote an issue in GitHub to the person who made this package. One of people answered my question by suggesting to update the version of R. Same like you, I was using 3.6.2 version of R and then I followed his suggestion, I updated my R version to 4.0.4 He also suggested me to upload "systemfonts" package before I upload the kableExtra by writing this code install.packages('systemfonts', dependencies = TRUE). Yet I think it was because of the old version that ı was using. So the bottomline is, if you update your R to 4.0.4, and run these 2 codes;
install.packages('systemfonts', dependencies = TRUE)
install.packages('kableExtra', repos='https://cran.ma.imperial.ac.uk/')
it is just going to work I guess. I mean it worked for me.
I updated my R to the most recent version 4.0 and tried to install the 'umx' package which worked fine when I had the 3.6.3 version. I then changed my R back to 3.6.3 version and the 'umx' package still won't install. I get the below error:

installing source package ‘kableExtra’ ...
** package ‘kableExtra’ successfully unpacked and MD5 sums checked
** using staged installation
** 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/3.6/Resources/library/gdtools/libs/gdtools.so':
dlopen(/Library/Frameworks/R.framework/Versions/3.6/Resources/library/gdtools/libs/gdtools.so, 6): Library not loaded: /opt/X11/lib/libcairo.2.dylib
Referenced from: /Library/Frameworks/R.framework/Versions/3.6/Resources/library/gdtools/libs/gdtools.so
Reason: image not found
Calls:  ... asNamespace -> loadNamespace -> library.dynam -> dyn.load
Execution halted
ERROR: lazy loading failed for package ‘kableExtra’
removing ‘/Library/Frameworks/R.framework/Versions/3.6/Resources/library/kableExtra’
Warning in install.packages :
installation of package ‘kableExtra’ had non-zero exit status

The downloaded source packages are in
‘/private/var/folders/88/d4_sv_l174vcbkn5f8r6ytjc0000gn/T/RtmpSb59yk/downloaded_packages’
Not sure why this is as everything was fine before I updated my R, and my expectation was that it would be fine again when going back to my original R version but this is not the case. Any help with this would be greatly appreciated!
Thanks,
Rionagh