Flight Simulator X: Steam Edition won't run because "not enough disk space available"

I downloaded Microsoft FSX on my external SSD to run on Parallels. However, when I try to run the game, it comes up with an error saying:

"Your computer does not have enough disk space available to run Flight Simulator. The program will now exit."

I've been trying to find a solution all over the Internet but to no avail. I'm thinking this has something to do with how Parallels only automatically adjusts its hard disk space, using the existing macOS drive, and doesn't run by partitioning.

Thanks a lot!