"Humanoid avatar must have head, hands, and feet bones mapped. Your avatar is humanoid, but its feet aren't specified! Your avatar is humanoid, but its upper arms aren't specified!”
If any humanoid/animator bones of your armature are renamed differently than their original names in the FBX rig, the VRCSDK will be unable to find those bones.
May also occur if your FBX rig cannot be detected via the Animator component above your Avatar Descriptor on your Avatar Root.
Revert any re-named bones to their original names from the FBX.
Also make sure that you have your avatar mapped on the Animator above your Avatar Descriptor on your Avatar Root.
Afterwards, check to see if you have the My VRC Avatar Descriptor is not showing the Playable Layers properly / Double FX Bug
If you have import errors, look over Ambiguous Transform ‘Head’ found in hierarchy for human bone ‘Head’