The auto rigger can't map my existing skeleton, but my model has no skeleton

So I've been trying to get my FBX to work with the auto rigger but am not having any luck. It was previously rigged with mixamo but I've removed the armature and all the vertex groups and am trying to rig it with mixamo again with no luck. The auto rigger still thinks it has a skeleton. I'm attaching the model to this post, can anyone help me out with this? cheers