Can't subscribe to Game Pass Ultimate

If you are already a Game Pass Ultimate subscriber, then any Gold codes you redeem will be converted to Game Pass Ultimate. If you are not an Xbox Game Pass Ultimate Subscriber, then no, the Gold codes should not convert to Game Pass Ultimate.

I haven't had ultimate but I had normal game pass for pc before

That would be why you can't redeem it then...from the FAQs in the link I posted before:

"If you were previously subscribed or currently subscribed to Xbox Game Pass for PC, or an Xbox Game Pass Ultimate subscription, you will not be able to redeem the two month trial for Xbox Game Pass Ultimate!"

Hi the Game Pass Ultimate is the pack of this Services: Game Pass(Xbox) + Game Pass(PC)+Live Gold

If you only play in Xbox, Subscribe the Game Pass Ultimate is the Same with you Subscribe Game Pass + Live Gold but you only go pay $ 15.00 vs $20.00 by Subscribe Game Pass and Live Gold Se-pared

As mentioned by others, Game Pass Ultimate has Live Gold, Game Pass console and Game Pass PC and therefore it will not be possible to subscribe to Game Pass PC individually, while it has Ultimate. If in the future you stop being an Ultimate subscriber,
then you will be able to take advantage of specific PC offers.

Hello!

"Trial" means you can use them on your account so long as you're not subscribing / have subscribed to the service in the past. So if you have used the Game Pass Ultimate service in the past, for instance, you can't use a trial code for it on the same account,
it won't work.

And no, Game Pass Ultimate is unique, it cannot be considered as either Gold or regular Game Pass. But you can use Game Pass Ultimate to get the same benefits of the other two services in a single package, for sure.

Hope I helped!