I am playing as the count of Breifne in Ireland. My chancellor has fabricated a claim on the neighboring County of Connachta. The count of Connachta is also the petty king (duke) of Connachta, although holding only this county.
My idea was to now somehow press the fabricated claim, but I don't know how to do this. I can not declare war on the petty king. The tooltip over the declare war button says that I am fullfilling all requirements (including the causes belli) but one: The petty king "does not hold any of your close relatives hostage".
How can I press my claim and declare war on the county of Connachta?