I just downloaded this mod and the quest played fine up until the point where you had to check with the orc foreman in the sewers. When I enter the sewers, however, I get a popup message saying the game is unable to load "in_moldcave_doorway00.nif" Is this a texture in the vanilla game? If not does someone know where I can get it/how to fix this error? Thanks!!

That is a mesh, not a texture and it is from the vanilla game.

in_moldcave_doorway00.nif is indeed a vanilla mesh, but as you surmised, that message is usually due to the game not being able to find the proper texture. The textures the vanilla game is looking for are:

tx_natural_cavern_wall20.tga
tx_wood_weathered10.tga
tx_natural_cave_beam01.tga

In a default install, this mesh and the associated textures are found in the BSA, not in the actual meshes or textures folders. If any mod or pluin-less texture replacer replaces any of these textures, you may see the nif in Meshes\i folder and/or files of the same names as the three above in the texture folder with any of the following extensions: .bmp, .dds, or .tga.

If any of those files exist, one or more texture files may be missing, corrupted, or the path isn't correct.

The fastest and easiest way to fix this is to change the extension of the offending files to .OLD and go on about your business. This will force the game to get the default mesh and textures from the BSA.

If you are up for some detective work, you can search for those files in any replacers you may have and see if you can find a missing texture or wrong file path in the nif.

Good luck!

Thanks for you help!! I went searching through the files on my hard drive and found tx_wood_weathered10.tga in my textures folder, but did not find any of the other files you mentioned in my meshes/textures folder. I changed the extension to old but the same error came up Is my only solution to reinstall?

No! Don't reinstall! I had the same error. It's the mod's fault. They are looking for the mesh in the wrong location.
The easiest thing to do is find "in_moldcave_doorway00.nif" in your "Data Files/Meshes/i/" folder and copy it to "Data Files/Meshes/"
That's what I did and it worked.

The file isn't in my meshes\i folder though :/ Would it be hidden anywhere else? I've never moved any of the files in there much so I don't know what happened.

It's inside the BSA; Morrowind does *not* extract all the files in the BSA into the folders. The only way those files could be in any folder is if the mod put them there. Reinstalling Morrowind would not affect anything. Re-installing the mod might, but only if the in_moldcave_doorway00.nif is in the archive somewhere. I would have to d/l the mod to be sure either way. Helseth is right, the mod seems to be calling for a file that the archive didn't install.