Before uploading to Steam (if doing it via the devkit), check the output folder in ARKDevKit\ModTools\Output. Does it output all of your custom files or pretty much nothing at all?
I had that problem. Turns out, at present you have to specify a map for the mod, and I think it needs to be in the mod’s root directory. An easy fix would be copying the map from “genericmod”. It also needs a unique name, so rename it (and don’t use “testmap” since that already exists somewhere).
Not sure if that’s the problem you’re having or not, but try it.