Try uploading it from a mesh part?
I did that, so idk what else to do
It could possibly be the size of the mesh, or possibly the name of it?
yes, they said something about that
Im quite new to blender so idk how big stuff is meant to be lol
Its something about, you’re not allowed more than 20k vertices, I think
Shall I show a screenshot of my build? Its simple but idk if it breaks any of the roblox rules for it being too big.
So I actually dont know what the problem is
the default cube has only 8 vertices
Its most likely the size of the mesh 
like that? Im not sure Im making a sim so this is quite crucial.
If you click A on your keyboard, it will select all of the vertices, faces, etc. Then the amount of vertices should show up
ok give me a couple mins please if possible
Im not sure, it could be the name of the file. Or the size of it? Never tried that before
Try uploading the mesh from a different place/game. As far as I know, such an error has nothing to do with the count of vertices. (Besides that, the limit is 10k and not 20k).
If this doesn’t help, try re-saving the mesh as a different file type (.fbx or .obj). This should help.