I'd like to start with the fact that I'm very new to blender overall, and am doing this mostly for fun/as a hobby.

I downloaded a raw Roblox model rip obj file from sketchfab and I manually repositioned the mesh parts, rigged, and textured it. This was my first time doing all of these.

My process went as follows; I moved all the individual parts (the mesh was split into many separate pieces) into a T-pose shape. I grouped up all the individual meshes into one. I then built an armature. I went on and selected all the vertices of each individual part and assigned them to a vertex group, to which I assigned to each corresponding bone. Finally I cleared a list of materials that came with the original obj, I made three image materials for three separate parts of the model, to which I uploaded a few PNG's, and I assigned them to the mesh via all-selecting (ctrl + L) the vertexes of each individual part of the model.

While it did come out successful (I was able to go into pose mode and move it around and such, and also see the applied textures in the material preview viewport), it seems to have an error when trying to export it via the blender source tools. As seen here; https://imgur.com/a/kFVwedG
(The sketchfab post in question: Sketchfab link [sketchfab.com] )

Would anyone happen to know the issue here? Or how to fix it?