This may be a dumb question but here goes. I bought a used camera on craiglist and it came with a Ravelli pan-tilt tripod. I haven't been able to figure out how to mount a camera on it. There is the screw in the middle of the head for screwing my camera's mount on to, but on the head there is a metallic piece a half-inch from the screw that interferes with my camera as I'm screwing it on. It's small and silver and I can press it down but it's hard to press down while screwing the camera on.

Can someone please tell me what is this metallic piece and how am I supposed to get around it to screw on the camera? Here is a photo of the head with the screw and metallic piece:

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