This is probably the most naive question ever:

I've rotated a GIMP image and want to move on. I see no NEXT or DONE "key". The keyboard's ENTER key does nothing and I cannot find anything to complete my ROTATE operation. Surely it requires just a single keystroke, applied somewhere, but how the devil do I find it? The GIMP tutorials for beginners that I have examined don't show this most basic step. Would someone please enlighten me.........in a kindly, tolerant instruction? ---Oldster, 86 and counting

Did you go through these? When you click rotate button it's "done".

https://docs.gimp.org/2.8/en/gimp-tool-rotate.html

2.10

https://docs.gimp.org/2.10/en/gimp-tool-rotate.html

Welcome to Gimp Chat.
Because there is an almost limitless number of operations you can do to an image, any operation is only 'done' when you save the image. Gimp uses 'xcf' as the working file format but for distribution of your images use the 'export' function.

Maybe you rotated the image while having a selection. If you did, then you have a floating selection. Click on the little Anchor icon at the bottom of the Layers dialog.