GOG;Steam supposedly has no storage space?

Hello, I have a problem with Steam and GOG. When I try to save my game on GOG, I get the message "Could not save, not enough storage space." It's similar with Steam; when I try to download from Steam, it says "Not enough storage space." However, the specified target drive still has a good 1.5 TB of free space.

I can't explain this and can't find a solution