Well, I gues you did given you got that error :P.
Anyhow, I went searching for the cannot allocate error and found this thread: Cannot resolve the "cannot allocate vector of size … " error - #57 by bgoodri
It looks like the discussion in that thread is around cleaning out old Makevars and R environment stuff. Like, delete all your Makevars.win, .Renviron, etc (or save backups), and then follow the Windows 10 instructions here: R 4.0, rstan, and you