Sims 4 custom content bald hair

When the hair shows up as bald it is because you are missing the mesh. You shouldn't have to build the mesh yourself, usually when there is a hair that requires a mesh there is a link to the required mesh & you just install it the same way as the hair.

Which custom hair did you download?

Also, I know whenever the game updates the hair I downloaded resets to bald as well and I have to manually go back to edit the appearance & change the hair back. A couple of the hairs I downloaded showed up as bald even though I included the mesh which I'm assuming was due to there being an issue with the file or something because the rest of the hairs worked perfectly fine.