Why is my Blender fbx mesh not importing into Roblox Studio?

So I made my first terrain model, I inserted a MeshPart into Roblox Studio, and imported my fbx mesh file.

Errors:

When I import it I get an error:

"Error while creating named asset: Could not upload MeshPart"

and sometimes I get this error:

"Unable to publish MeshPart: Inappropriate name"

...even though the name is just "Mountain" and "grass"

EDIT: OK, so I noticed that I can import my "Grass" model, and that is probably because it is a lot smaller than the "mountain" file. Also, I noticed that the mesh itself is actually REALLY big.

So the mountain file didn't import because it was too big. Then I agreed on making the mountain mesh smaller because it was too big (a window asking to make the mesh smaller pops out when I import the mesh). But it still didn't work and I got the same errors!

I read a little bit on the dev forum, and it said that I have to select everything in blender and export it. I did it before the results I described before.

SECOND EDIT: I made my mountain mesh smaller in blender and saved it as a new mesh. It still didnt work even though the mountain was a lot smaller

Kindly explain what is the problem if you know the solution, and ask anything if I missed on something or explained it wrong!

Thank you in advance