Usually a sign of a corrupt texture. Make sure the texture path matches the path specified by the model. If it is a custom BLP texture then it may be corrupt and need to be re-built.
Rebuilding the texture should be as simple as loading it in BLP lab and saving it as BLP1 again while making sure the correct number of mipmaps are generated. If the correct number is not generated (at least 1 is 1x1 pixel) then increase the number of mipmaps until BLPLab does bother to. Otherwise if using the current version of Warcraft III you can also export the full scale image in a common format using BLP lab, import it into GIMP, and then export it as a DDS file with a supported encoding (DXTC 5). This then goes in the same file path as the expected BLP file except with the extension changed to .dds
.
If these are old custom models that relied on standard skins to work then it is possible the skins they require no longer exist in the game data. Some classic graphic assets were removed from newer versions of Warcraft III for some reason, possibly due to licencing, copyright or censorship concerns. In this case you need to obtain a legacy version of Warcraft III which does have the skins, export them and then import them into your map at the expected file paths.