The main thing I remember that causes this is a missing folder structure:
Data\Editor\Mods\YOURMOD\Levels\YOUR LEVEL NAME
Usually there's a file file in there called Terrain_000.data (or 001,002, etc)
So the question is, why isn't that folder being generated? Assuming that's your issue anyway.
Are you certain you are using the EE editor and not the old classic one? Because the classic one used to have this problem.
First thing I would do is check your datapaths in Options>Preferences
Should be something like this:
EditorDataPath:
C:\SteamLibrary\SteamApps\common\Divinity Original Sin Enhanced Edition\Data\Editor\
GameDataPath:
C:\SteamLibrary\SteamApps\common\Divinity Original Sin Enhanced Edition\Data\
Second thing I would do is just go looking around the folder structure yourself and see if the folder ended up somewhere weird:
Data\Editor\Mods\YOURMOD\Levels\YOUR LEVEL NAME
Lastly, just to get it working again I think all you have to do is manually add the folder structure yourself if it's missing.
So just go to Data\Editor\Mods
And create the following folder tree:
YOURMOD\Levels\YOUR LEVEL NAME
Make sure you get the names exactly right. The YOURMOD name will look something like this with characters at the end:
A_Necromancer's_Crusade_7c57ebf1-6a07-47fc-99e7-8131a2d5ca05
If you don't know what yours is supposed to be, you can see it in the folder:
\Divinity Original Sin Enhanced Edition\Data\Mods
Should be exactly the same folder name as the one there.
