When you open png or jpg from web & you want to edit locked background layer. Then you double click on ‘background layer’ You get this warning called ‘Could not edit the layer because the layer is locked.’
Here is How to fix this error
Select Background Layer
Double click on locked layer which will bring new layer dialog box click OK & your background layer will become Layer 0 and is unlocked
Option Click on Mac or Alt Click on Windows on Locked layer to make layer unlocked
Drag Layer’s Lock Symbol to trash icon available at bottom panel.
Photoshop will convert background layer to Layer 0
Now layer will be available for editing & manipulation.
Hope it helps,
Thanks & Regards