If a mod overwrites a vanilla texture .bsa file, the textures from that archive display as missing (pink) in NIF Preview.

[Ghos71y]

For example, if you have Cleaned Skyrim SE Textures installed, all vanilla textures (that you aren't overwriting with a loose file) will show as missing.

This also happens when overwriting the vanilla .bsa files with a copy of the (vanilla) .bsa installed via MO2.