Making a Maptheme, getting an Error

[DarkMalady 0]

I have been trying to make a new theme, to go with a specific map I have been working on.
I have been making 4k textures, and everything has been going well enough, until it came time to add in Cliff and Sand normals.

it keeps giving me the same error, " Something went wrong processing textures"

I have tried changing the resolution of the .png file, the bit depth, loading skylines in -noWorkshop mode, even tried saving as .jpeg
always the same (un-informative) error.

the Normals were generated using https://cpetry.github.io/NormalMap-Online/

from my cliff and sand textures.