Hi, I'm a Linux user (Fedora 31) and here's what worked for me:
I ran install.packages("lme4"). First attempt did not download successfully so I tried again. The download succeeded but I got a different error message:
ERROR: failed to lock directory ‘/home/isaac/R/x86_64-redhat-linux-gnu-library/3.6’ for modifying
Try removing ‘/home/<username>/R/x86_64-redhat-linux-gnu-library/3.6/00LOCK-lme4’
I removed the directory. My next install attempt successfully installed lme4.
Subsequent installation attempt for car installed the dependency pbkrtest and finished successfully.