And I apologize - I realize I did not answer the rest of your question! I am running on Ubuntu 18.04.
Also, I did get the following message when I attempted to install the packages in R, as I am running this on a shared server (and R is installed in a read-only directory):
Installing package into β/software/R/3.5.1β
(as βlibβ is unspecified)
Warning in install.packages("nlme") :
'lib = "/software/R/3.5.1"' is not writable
Would you like to use a personal library instead? (yes/No/cancel) yes
So I have installed the packages to my personal library, but would this be causing some problem with reading the packages? Thanks!