I am working on a Unity scene and as the 3D model is missing the bones, I ended up going to Blender for the rigging.
After exporting from Blender (in .fbx) and importing it on Unity, I went through the configurations (as @vcard suggested here)
In the "Rig" section, I pressed the button "Configure" (to configure the Avatar Definitions) and came across an error "Character is not in T pose".
I solved this one quite fast by accessing the menu "Pose" (in the Inspector) and then "Enforce T Pose".
After that, back in the scene, I came across the following error on the Inspector:
Invalid Avatar Rig Configuration: Missing or invalid transform: Required human bone 'Head' not found.
I went straight to the configurations and decided to add the specific bones to the Head that appeared on the inspector (in the Avatar Definitions configurator). With that, I came found out that the problem is really the organization where there were placed by Rigify.
Any guidance is appreciated.