I'm attempting to extract the AM sauna towel mesh for a project, but when I try to use SimPE I get an error that it cannot find all parts of the mesh. Am I doing something wrong, and is it fixable?

Some meshes don't extract using the extraction tool. It could be that SimPE is not looking in the right place... but sometimes the extraction just fails.

The easiest thing if you're really stuck is to look in the game files with SimPE. All you really need is the GMDC (which is the part you edit in MilkShape and then reimport into your mesh project). You can find the game files in... let's see if I can remember... C:>Program Files>EA Games (or Origin Games, if you're using the Ultimate Collection)>The Sims 2>the EP or SP where the mesh is from>TS Data>Res>(Sims)3D>Sims03. I hope that's right. All the GMDCs for that EP or SP will be in that particular Sims03 folder. Just open up that folder, find the GMDC you want, and right click on it to extract. Then you can alter it in your 3D program (such as MilkShape).