In Gimp 2.9 I want to add an Alpha Channel (Layers>Transparency) I expect to see a layer of transparency beneath my main image. However t no new layer of transparency appears. Its in RGB mode. What to do?
The alpha-channel is a property of a given layer. Adding one allows you to make transparent pixels later (for instance with the eraser), but doesn't make your layer transparent immediately.
The alpha-channel exists when the image is loaded from a format that allows transparency (such as PNG...). It is also added automatically to all new layersbecause it rarely makes sense to stack opaque layers (except for GIF animations).
There is no such thing as a "layer of transparency". The checkerboard pattern isn't a layer, it it below the lowest layer, you see it where all the stacked layer are transparent.
I am currently using Gimp 2.10 but if I remember properly I think you should select adding a new layer, whatever you choose will be right and then you will realize that adding an alpha channel option has been enabled.
If you're wondering why it seems like you magically get alpha channels sometimes, it looks like GIMP will add an alpha channel automatically when you add a layer (even if that layer doesn't have a background of type Transparency); I suspect that GIMP uses alpha channels for its internal use. E.g., this caught me off guard when editing a single layer grayscale image, then it silently converted my image on save to an RGB with alpha image. Check the channels tab if you are ever in doubt.
Hope it helps.