Background layers behave a little differently than regular layers in the Layers panel; they’re locked in position and can’t contain transparency. Also, there are some things that can only be done with a Background. If your document no longer has a Background layer, don’t worry: You can change any layer into a Background layer. Bear in mind, you can only have one Background in a document, and it has to be at the bottom of the layer stack. Now we’ve established the rules, choose the layer that you want to convert in the Layers panel, and go to Layer>New> Background from Layer. Any transparency will be filled with the current Background color in the Toolbar. 

This tip previously published in the December, 2020 issue of Photoshop User magazine in Colin Smith’s “Photoshop Tips” column.