Why can't I buy any games in Uplay after linking Steam account?

I had to learn this myself also, apparently if you ONLY have Steam games on your Uplay account, the UPlay client expects you to only play said games, and thus, only pruchasing games through Steam. It's an easy fix though.

Go to https://store.ubi.com/ and login, here you can purchase new titles. When you first buy a title here, it'll unlock your account and you can access things like the store through the client.