app store won't let me download apps

I tried installing an app from the app store but it says "could not complete your purchase. cancelled". That's all it said and it won't even tell me what is wrong, or why the app can't be installed. I tried downloading other apps too, but the same note popped up. I was wondering if it's because I haven't updated to macOS catalina, which is because I read that it has compatibility issues with adobe illustrator. I use illustrator for school, so I don't want to risk messing it up by updating my OS.

I have tried these methods already, but it didn't help:

1) signing out of my icloud account and signing in again

2) changing my icloud password

3) restarting my macbook many times

4) installing the app in safe mode