Forest editor won't work ?

that probably happens because there is no forest brush group. there is currently no way to add it from the forest editor.
you can either:
1. base your map off of one of the existing maps that already have a forest brush group, for example the template map or
2. add a forest brush manually by clicking on the plus symbol, then clicking on "other classes", look for "Forest Brush" and then click create. To make this work in the forest editor, you should save the level and then reload it

 

can you create forest brushes on official maps? either way, please recreate the issue without any mods and create a support id https://supportid.beamng.com/