Can't activate Ultimate Game Pass

this is still half answered. I am at 17 tenure and want to keep it. I am somewhat weary of upgrading to ultimate game pass as there is nothing super clear about tenure

it seems there would be some kind of gap when going game pass ultimate -> Gold, as you can't convert back? or is it covered if i were to cancel game pass ultimate during a mid-month of active subscription, and same day/before end of active game pass ultimate
expires - add active gold sub??

And that's why you can't access the game, you don't have currently a Game pass ultimate suscription active on that account

I have an active xbox game pass ultimate and I'm logged in on the correct profile, but I find that I can't install any game not just forza.

You need a credit card.
If you can't, does an error message appear?

Try this link: https://www.xbox.com/en-US/xbox-game-pass

Nope, you need:

Never had Game pass ultimate before

Have a valid credit/debit card in place in your billing information

Don't have auto renewal activated on other subscriptions as Gold and Regular Game Pass