Create subjects - what is required

[Jupiter]

So I have a province with core on it. It belongs to Guyenne and says "considers this to be core...1494 this province will have slipped". And I can't create Guyenne as a vassal.
The core lists Guyenne as having a core on it.
So my question: what is needed for creating a vassal.

[Kagemin]

The only other requirement is that you need to own at least one of their cores that also has their primary culture. Iirc there's one Basque province in souther France, if you only have that one that might explain it.
Only other thing that comes to mind is that you can't release them when one of their provinces is occupied by rebels, or while you're at war.

[Dreamers]

Being at peace, a core of the vassal that is not being sieged, that they don't exist and that at least one of the provinces you are releasing them with has their primary culture.

[Jupiter]

Primary culture... Riiight, I just had to take that core didn't I, said it was close did they...