I made a custom rig with a union for a head but for some reason I cannot select it when I am animating. It is really weird and any support is greatly appreciated. More information may be provided if needed. Not sure if this is in the right category. All parts are unanchored and massless.
I am wondering if it has to do with the hitboxes or anything.
Hi! This would probably fit better in #help-and-feedback:art-design-support as I believe that category includes animation + custom rigs, and stuff like that. 
Ah, thank you for that. Totally didn’t see that category.
Needs more details if you’re looking for support, such as a picture of the hierarchy and some explanations of properties enabled (e.g. if any of the parts are anchored).
Added that now. It might be because the head is a union but I am still unsure.
You need to weld all the parts of your rig with together with Motor6Ds that have unique names (they shouldn’t be the same). Motor6D is a joint instance that holds parts together but also allows for them to be animated. This also registers the joints in the Animation Editor so that you can configure them.
You should select the head by using the + sign on the animator, if it’s Roblox’ one you’re using
I used RigEdit Lite to make Motor6D’a for the rig. It put them in the head though.