I have installed, uninstalled a couple of times, but still get the same issue. Windows 7 and QGIS 3.6.1 and latest versions if R and R Studio. It used to work perfect until I changed computers.
R execution console output
Warning in install.packages("sf", dependencies = TRUE) :
'lib = "C:/Program Files/R/R-4.0.3/library"' is not writable
Error in install.packages("sf", dependencies = TRUE) :
unable to install packages
Calls: tryCatch ... tryCatchList -> tryCatchOne -> <Anonymous> -> install.packages
Execution halted
C:/Users/your_username/R/win-library/4.0
. I suggest to use it in the provider setting, too. (Then the libraries downloaded in R Studio becomes automatically accessible from QGIS). – Kazuhito Jan 12 at 8:26