When we open up an image without an alpha channel in Gimp, it is not be possible to delete the background like users would intuitively expect; this behavior is confusing and non-intuitive to users, and it does not appear to add any value to the editing process.
To fix the problem, users will have to spend time manually adding the alpha channel. This is done by going to Layer → Transparency → Add alpha Channel
Why alpha channel is missing in Gimp
Because an image might not contain an alpha channel, users will have to add it manually. A better way to solve this problem would be if Gimp automatically added an alpha channel when users attempted to delete part of an image; as the word indicates, delete means to remove something, not "fill with background"!
Often users will notice that they can not easily delete part of an image, and just have it filled with an empty alpha channel.
How to add an alpha channel
There are two ways to fix this, either we have to right-click the layer containing the image and click on Add alpha Channel, or we must go to the layers menu and add the alpha channel from the transparency menu. Below screenshot shows how to add it from the layers dialog:
Alternatively, we may also add it via Layer → Transparency → Add alpha Channel: