I could just go get a free Maya export to learn Unity's rigging but I know it inside out now because of the many different attempts to get this to work. My process in this effort has not gone well. The mapping shows a green T-pose but when one switches to muscles tab the model crouches into a loose fetal position and the skeleton follows this and is gray. The rig tab had the possibility there but when I changed the Muscles and setting parts the changes never were saved even though Unity reports that there is a skeleton colored gray. The Animations are only transform modifications and there is not skeleton for IK.
I have just encountered this with my FBX and here is the journey. Your FBX is incomplete or not compatible with the import process.
