I've encountered this error whilst trying to upload my 3d model here:
I cannot figure out what is causing the error; I've tried to disable materials and it did not work. I'm very new to Blender so if possible, providing an explanation would be of great help!
remove the bones
move the camera and lights out of the model
export as Fbx and upload to Mixamo... you can put bones\ colours back in after Mixamo riggs it if you want them
This helped tremendously, thank you so much!!