Pre-rigged blender character - "Unable to map your existing skeleton"

You could try retargeting the outcome from Mixamo to your rig or simply adjust Mixamo outcome and work on the weight painting.

Mixamo will only animate a Mixamo bone set... if you want to convert Mixamo rigs \ animations then I suggest you use professional software like Iclone + 3Dexchange

Mixamo reads that your model has bones and so stops... that is the correct step because otherwise it would remove your own bones

if you want Mixamo bone sets then export as wavefront Obj and upload that to add animations