Can terrain be turned on after adding location in the browser version?
No. You only get a flap map in the web version. You’ll need the desktop Pro version to get terrain and imagery.
In the free web version ‘Add location’ adds the selected map region as a group with as layer ‘Location Snapshot’ assigned to it.
You’ll need to import or create your own terrain which you then could have the layer ‘Location Terrain’ assigned to it (or any other name).
There isn’t a ‘Toggle Terrain/Snapshot’ button in the web version. But you can turn on/off layers manually through the ‘Layers’ window.