Quote Originally Posted by PatricianS View Post
Unfortunately this error seems to appear whenever you make a requirement for something and then in the same line make it a "requires not"... It happens with hidden resources as well as with the same building tree :(

Guess that's the only options left then... Thanks for the answer :D
Make a hidden resource that is not present in any region and add an 'or' condition to the end of the requirements line. Like this:
Code:
requires building_present_min_level treeX level_2 and not building_present_min_level treeX level_3 or hidden_resource nowhere
I use this for filling in all sorts of "gaps" in the building DB. I tested this quickly, and it all seemed to work as intended with no error message.