Help:Troubleshooting/Map Issues/Graphical Glitches/REDWALL sky
Levels search for the SKYxx texture that matches up with the map number (e.g. MAP51's sky is SKY51) unless said otherwise in the Level Header. If your map is in a free slot (it's not used by normal SRB2) it searches for a sky texture for that number, even if one does not exist. Try creating a new texture, or change the SkyNum entry in your level header. Did this solve your problem?