Help, Rig parts/bones not showing up in Roblox Animator

Hello, I use the Roblox Animator to animate my Viewmodels for my custom gun system.

I use skinned meshes for the viewmodel arms:

When I press the plus (+) button to add a new bone/part, it refuses to add the bone/part into the list.
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Here’s the Viewmodel’s Children
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This makes it really hard to animate, the animation still plays as normal in game when saved to Roblox but it’s horrible to animate. Please help.