Fill function is not available.

The problem is going to lay somewhere in that long list of layers and groups.

You have various options.  The best way would be to locate the layer that black shape is on. If you select the Move tool and Cmd click anywhere on that black area, then that should select its layer.

You could Cmd click the layer to load it as a selection, and fill that selection with your colour of choice on a new layer.

Or you could lock its transparency, and fill with colour.

I would say that if you are fairly new to Photoshop, then it might be better to learn the basics aboput layers, groups and layer masks on a more simple document with just two or three layers.  Give it a go, and come back if you get stuck.