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"How do I apply the layer mask? I can see from the menu Layer - Layer mask - Reveal all or Hide all."

You can also invert the Mask by holding the Alt key and clicking on the Layer Mask Icon

[ATTACH]50008[/ATTACH] Layer Mask Icon

"The opacity of the layers panel won’t change, when I try to change it  from the 100% to see what happens  it just goes back to 100%  again"

Brightness and Contrast Adjustment over done just for demo

[ATTACH]50009[/ATTACH]

Effect after lowering the Opacity

[ATTACH]50010[/ATTACH]

I sure ALB68 and Tom Mann will give you detailed instruction on how to achieve your goal, hope my bit helps a little