Thanks to a little out of band chat with Josh, we discovered the issue was, indeed, a path issue. For some reason the path to R was not in the PATH. So I ran this from within R:
Sys.setenv(PATH = paste('/opt/R/3.4.4/bin/:', Sys.getenv("PATH"), sep=''))
Which worked because I am running R 3.4.4.
FWIW, my path before running the above command was as follows:
> Sys.getenv("PATH")
[1] "/home/rstudio-user/bin:/home/rstudio-user/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"