Why can't I get a SINGLE photo from a burst in Photos (on Mac OS High Sierra)?

Does the procedure given in the help (quoted below) not work for you? It worked for me in a quick test. (I underlined the bits I forgot the first time I tried it.) Where does it fail for you?

"When you open a burst, you can view all the shots in the burst and select your favorites to work with individually.

1. Double-click the burst in the moment or in the Bursts album in the sidebar.
2. To select photos from the burst, click the Make a Selection button in the upper-left corner of the photo.
3. Click each photo that you’d like to save as an individual photo, then click the circle in the lower-right corner to mark the photo as selected.
4. After selecting the photos you want to work with individually, click Done.
5. Do one of the following:
   • Keep the selected photos and delete the others in the burst: Click Keep Only Selection.The selected photos appear individually in the moment and unselected photos are placed in the Recently Deleted album. The burst no longer appears in the moment or in the Burst album.
   • Make the selected photos appear individually and keep all of the photos in the burst: Click Keep Everything.The selected photos appear individually and the photo burst remains unchanged.

2. I cannot get a SINGLE photo out of that burst to save anywhere.

- Online instructions are useless (i.e. they don't work)

That is true. It is not possible to do anything with burst photos, as long as they are stacked as a burst - you have to convert the burst to single photos, before you can use the photos from the burst. This is very inconvenient, but the way it works.

Try to select at least two photos from the burst, even if you only need one, and select to "Keep All". And then delete the extra single photo that you do not need from the moments. I cannot test right now on my Mac, but I remember that I had occasionally that problem on macOS X 1013.2 , that I needed to select at least two keepers, before any were photos were saved from the burst.

After step 3, did you get the checkmarks in the circles and on the array of thumbnails?