Can't export VTF files

[Octobyt3]

I'm trying to make a custom texture for a model for Gmod, but it has a alpha mask layer that I need if I want it to work. Found a way to properly retexture it in GIMP, but when I try to export it an error message pops up with the error code: '# error_layergroup_wrapper'. I've looked evrywhere online, but there is no results for this error. 
Why does this happen and how can I fix it?