why can't I make my client states independent?

[Mamluke]

Playing as Spain in Ironman, I'm emperor of the HRE, I'm trying to at least get the 3rd reform for the -10% core cost, but I would like to get Revoke The Privilegia if possible.
there are only 3 free cities and since nobody wants to became one, I decided to make 2 client sates with the republic government, grant then independence and make then free cities.
However, the button to break Vassalization is grayed out and I don't know why, I created then like 7 in-game days ago, I'm at peace and vassal Deli seams to work fine.
Please see the Images for reference.

 

[bbqftw]

You can't break vassalization while at truce with them? or does that apply only to vassals gained through wars?

In any case, you can solve this by 100%ing a country and forcing them to accept you ending their vassalization.