From your description this seems unlikely to be a Numbers problem per se. To test you might try saving a Pages document and seeing if you have the same problem there.
If you haven’t done so already you may also want to do some basic troubleshooting, restart your Mac, sign into another user account on your Mac and try saving there, etc.
Another step you might consider is checking your disk with Disk Utility (do Spotlight search to find it).
Also you might double check to make sure you have updated Numbers and macOS to the latest versions that will run on your hardware.
SG