Answered
Why can't I unlock the layer?
Why can't I unlock the layer?

Why can't I unlock the layer?

This is an easy one thankfully. You are in the Index Color Mode which doesn't allow layers. It's often used for GIFs, etc.
Go up to the menu and choose Image > Mode > RGB and now you'll have access to unlocking the background layer by converting it into a normal layer (just double click on it then)

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