This table only allows for a limited number of items () for Apprentice accounts.
 

Does anybody know what this means ?

I didnt think there was a limit on the number of apprentice accounts that a patron can have connect to (or I can't find anything specifically) ?

If you're seeing this in a situation where you're trying to set up campaign sharing, it means the player isn't actually using a campaign character that gets the benefits of your patron account - they're using their own character, within their own account.