Do you have multiple screens? Maybe the “Add Tree” window is somewhere else.
If you launched 2021.2.17f1, it is advisable to update the tilemap package.
Are there any error messages in your console after you clicked the “Edit Trees” button?
If nothing helped, I’d suggest to delete the Terrain game object (not your terrain mesh!). Then uninstall the Terrain package. Close Unity. Delete the Library folder in your project folder. Launch Unity. Install the Terrain package. Create the Terrain game object, create a very simple terrain for testing purposes. Try to draw trees. If that worked, reuse your Terrain mesh.
For testing purposes, you could also create a new project and test the terrain because, in rare cases, Unity components might be broken.
Please feel free to ask our helpful community of students over on our Discord chat server.
If nothing helped, please report a bug to Unity via Help > Report a bug (in Unity).