Hi @frederictardieu,
It was a while ago now, but given the complications we moved away from the idea of app specific projects for our customers. I sounds like from other discussions around the community that this might be an option in the future (see [comments], (Issue type schemes via REST - #7 by jpardobl), JRACLOUD-40141 & ACJIRA-440).
At the time I didn’t understand that apps can not access all REST resources. The latest documentation seems to call out more clearly that apps cannot access project category.
Hope this helps.
Cheers,
Callam