I have, Poland,Lithuania and Bohemia, 80% counties, and the gold,prestige and piety, and I'm Bohemian and I still can't make Wendish Empire bug? or do you have to be pagan with out it saying you need to be pagan?
3 king titles independt and at peace. 80% is in red so maybe 81%?
Try to get another county. Sometimes I get 80% of the required land and I'm still unable to create the title. You should be able to do it after getting OVER 80%.
Interestingly, I think it is possible to create Wendish Empire only with one king title.
Maybe I have missed something, but I had only the Kingdom of Lithuania. However... all de jure lands of the Kingdom of Poland became de jure lands of the Kingdom of Lithuania and I was allowed to create Wendish Empire. The King of Poland became a titular title which i could create at will but I did not create it.
Strange, but perhaps this is a loophole to the 2 Kingdom rule, then.
Usually to form any empire you need (at least) 2 kingdoms. Just as to form any kingdom you need 2 duchies. Then on top of that basic requirement there may be extra rules, including 81% for empires, 51% for kingdoms/duchies.