I'm getting the titular error:
mcfork(): Unable to fork: Cannot allocate memory
The issue might be exactly what the error message suggests: there isn't enough memory to fork and create parallel processes.
R essentially needs to create a copy of everything that's in memory for each individual process (to my knowledge it doesn't utilize shared memory). If you are already using 51% of your RAM with a single process, then you don't have enough memory to create a second process since that would required 102% of your RAM in total.
registerDoMC(2), for example, will set the number of parallel threads to 2 (if you are using the