Mixamo for posing models or

 If it doesn't work in mixamo. Make a little trick on blender. Open model > edit mode> press Z (transparent) > press B (make a square to select model) > press W > remove doubles > export as obj.... then try to upload the model again in mixamo... it should work
 
 https://forum.facepunch.com/f/fbx/qsvx/How-To-Rip-3D-Models-From-Games/1/?fbclid=IwAR3WT2Sfjyr46Rha27J4TsUt8in_2IOrEQc-WHFmpCxQKkphmhKdydXfirY