Why I can't grant a duchy to a count?

Gavelkind Succession has some mechanics to avoid you giving your heir all your titles except one before you die, so you can't game the system.

You cannot destroy titles subject to gavelkind succession (except titular titles). You cannot give your primary heir more than a single county. That is, you cannot grant them a duchy title or a second county. (You can, however, strengthen them by granting extra castles or tribes.) Furthermore, you can only give your primary heir a county that they are expected to inherit.

https://ck2.paradoxwikis.com/Gavelkind

But it seems like you are not trying to give your heir a title, but another character. Does the character in question have any traits disqualifying them? is the duchy a regular feudal duchy?