Why can't I take part in trio cups?

I wanted to take part in the Trio Hype Cup with two friends. We all have 2FA and at least 1200 arena points.

When we wanted to get ready, the (usually me) button was always gray and instead it said: Deactivated.

my friend and i played ps4 and our mate played mobile. (was it because of it?)

my question would now be in conclusion whether any of you know what it was and if so how to do it differently so that it works.

If you have any questions about the situation, let me know.

Unfortunately this tournament is not cross-platform.

This means that there are three lobbies, so to speak: Mobile + Switch, console and PC.