That's not an idiotic question! It's fascinating that you've had exactly the same problem as I did (I've never yet managed to finish the tutorial -- I tried twice).
I came to the conclusion that the first time it happened, it was because I'd messed about with my diplomatic relations with Catalunya (seeing what would happen if I claimed their throne by ancestry!) -- so naturally they weren't on speaking terms with me any more.
The second time I was careful not to do that, but I think I'd let the alliance with them lapse -- it has to be renewed every 5 years, and with the default message settings, news that the alliance has expired may only appear in the history window down below, or in a TINY window towards the right top of the screen. Since then I've been into Options (F10), Message settings, and made sure that all such announcements appear in a large window so they can't be missed.
The other possibility is that you WERE asked to support Catalunya against the Mamlukes, but it appeared in that tiny window I mentioned, so you just didn't see it. Again, the solution is to alter the message settings.
I wish you success replaying the tutorial! Maybe one day I'll try it again too...