Why is terminal telling me that my .rar file is not a RAR archive?

Your file is empty. Probably an unfinished download or just the wrong file altogether. You will need to go back to wherever you got it from and try to download it again.

In future, remember that you can always use the file command to determine the file type of a file. In this particular case, you would also have been able to see that its empty by running

ls -lh  "/home/superpajamarama/Desktop/PS2 Games/Futurama.rar"

which would have shown an 0 byte sized file.

Actually two things comes to my mind:

Either archive is corrupted, it can be checked

unrar t file.rar

or rar is password protected

unrar e -pYOURPASS file.rar