The helper bones used to help place the head geometries also require removal to validate correctly. While these extra bones do not include any skinning data, do not delete any of the facial animation bones parented within DynamicHead, as these include important skinning data that drives facial animation.
Remove the additional head bones by selecting them and deleting them in edit mode:
If required, toggle visibility on your Armature object. The bones display in the viewport.
In the Outliner, expand the character's armature object and find the Joints object that parents the bone structure of the character.
Expand the Joints hierarchy by holding shift and clicking the expansion dropdown.
In the viewport, select any bone and switch to Edit Mode.
Under the Head joint, hold shift and select all the head children joints except DynamicHead.
With the extra head bones selected, right click in the viewport and select Delete Selected Bones.