Plutonia 2 (Textures?) showing up as black?

Howdy folks,

First-time poster here so my apologies if this is an inappropriate location to ask, but I'm encountering some issues with textures in Plutonia 2. It seems as though some textures in Plutonia 2 (such as the sky) are showing up as entirely black.

https://imgur.com/a/NpEKgyT

Any ideas as to how I would go about resolving this?

Plutonia 2, which is intended as an extension to Plutonia (Final DOOM),
should be played with the IWAD Plutonia.wad not with DOOM2.wad.