Custom mask with Beardlib have different folder structure than mod_override.
Mod_override folder stats with units, but BeardLib mod can starts with assets or other name depending on what the author of the mod have specified.
You can get the mask on 'MODS' cateogry on buying mask menu.

On Ela other texture, the file is supposed to replace an existing file already in the mod. It's the ela_df.texture file you want to copy-paste, not the folder.
The path is in the assets folder of the mod, the texture location.txt also have the path you can follow.