/vr/ - Retro Games

So I'm giving a try at adding textures to wads for the first time, but I'm hitting a snag. I'm trying to add the slimefall textures from Doom 2 to a Boom format Doom 1 wad using SLADE, but every time I test the map with PrBoom+ it gives me this error and I'm unsure what I'm doing wrong. Not sure if it matters but it works in GZDoom.