Please attach a screenshot of the Real World Terrain window.
Maybe I'll see some kind of clue.

Please make sure you are entering the access token in the correct section.
There are two places for the access token, in the Elevation Provider and Textures sections.
If you need elevations from Mapbox, you need to enter the token in the Elevation Provider section.
If you need textures, enter it in the Textures section.
If you need both, enter it in both sections.