Can't erase adjustment layer in PSE 2018

fotografffic  wrote Click on the layer mask in the layers panel to make it active then try painting/erasing on the layer mask. That's what I did. If one has the levels adjustment layer active instead of the layer mask, then trying to erase will give you that flatten layer message. If I understand you correctly, I didn't make a layer mask. As I mentioned in a reply to my own post, deleting the preferences file solved the problem...but I don't understand what caused the problem in the first place. Thanks for your input.

To expand a little over Jeff's explanation, an adjustment layer is not a standard layer made of pixels; imagine it as the virtual result of applying the same formula (transformation) to each individual pixel from the layer (or layer combination) to which it applies. The advantage is that storing the formula is much lighter than storing the resulting image with all pixels. The logical result is that you don't 'erase' or modify selected pixels, the formula still applies to each underlying pixel. You can erase the whole adjustment layer (you erase the formula). But adjustment layers can have masks just like normal pixel (bitmap) layers. You can mask certain areas to delete or  tone down the effect of the formula. Classical use: you use a hue/sat adjustment layer to desaturate an image and get a black to white image... and you mask a part of the image to let the colours come through.