I have the exact same problem. Found in this other topic that we’re not the only ones:
After I imported a terrain created in Gaea to a World Partition level that had basic terrain, I couldn’t place or delete foliage anywhere.
It broke all other levels I have in the project as well, even though they are using a different landscape. I deleted the terrain but that didn’t fix it for any level.
I’m afraid I’m gonna have to start a new project because nothing has worked so far.
