There is no error message when I try to add on the website. It just plain doesn't work. When I try to add from my phone I just get "An error occurred Try again" screen.
Wait, if you're talking about the web library, you need to say that — otherwise we're always going to assume you're referring to the desktop app.
ISBN lookup often might fail from the web library. We do our best to keep it working, but there's no dedicated metadata service for ISBNs (unlike for DOIs), and the main source blocks requests eagerly. We'd need example ISBNs regardless.
But if you don't have a good reason for not using the desktop and mobile apps, you should switch to those.
Upgrade Storage
Nothing to do with storage. Zotero Storage only applies to files, not library data, and it never affects what you can save to the local app.
ISBN lookup often might fail from the web library. We do our best to keep it working, but there's no dedicated metadata service for ISBNs (unlike for DOIs), and the main source blocks requests eagerly. We'd need example ISBNs regardless.
But if you don't have a good reason for not using the desktop and mobile apps, you should switch to those.