signing in on iTunes Store problem

You are probably trying to use an iTunes version older than 12.6.5.3 that is no longer supported by the iTunes Store, and you probably also get an "0x80090302" error message. If you want to use an old version that allows you to manage apps, then try 12.6.5.3. Here's a thread about your issue: https://discussions.apple.com/thread/250783805. If you don't really care about managing apps, then you could simply update to the latest version of iTunes.