I wonder if the downloaded Gimp “Normalmap” plugin is the problem. I am on the 2.10.x version (recently downloaded) and the normal map does -not- show up on “Filters” - “Map” menu. I found it by searching for a command in the Help menu. It found it even though I did not install any plugin. It is possible the built in feature has better output than the downloaded plugin you linked to. So in Jen’s video tutorial, I just search for the Normal tool while she uses Filters->Map->Normal map. Just a thought.
Jen also points to a “9x9” option that is not there in the out-of-the-box Normal Map tool. The only variable I might change is labeled “Scale” which defaults to 10. 10 is pretty good. If I want the pattern to get more rough, I up the Scale (more pinks). If I want it a little more smooth, I lower the Scale (less pinks).