The warning about the R version isn’t actually what’s causing your problem. One of the packages that plm depends on isn’t installed. Here’s the key bit of the (rather opaque) error message:
Your next step is typically to try install.packages("miscTools") and, assuming that’s successful, then restarting your R session and running library("plm") again.
If plm complains about yet another missing package, you can try install.packages("plm", dependencies = TRUE) — compared to the default (dependencies = NA), this tries to install a greater number of dependent packages and can be quicker than going through all the missing dependencies one by one.