I read the YouTube help page Add an audio track to your video:
Add an audio track to your video
- Sign in to YouTube Studio.
- From the left menu, select Content
- Click the video you’d like to edit.
- From the left menu, click Editor
- Select Audio
and then
use the search filters
to find a new audio track. Click Play to preview tracks.
- When you find a song you like, click Add
. The song will appear in the editor.- Drag the added track to change when the song should start.- Drag the edges of the box to change the amount of the song that plays.- Use the zoom options
for more precise adjustments.- You can add multiple songs.
- When you’re finished, click Save.
I am stuck at step 4: I don't see any Editor 
icon/option in the left menu or anywhere else:
"Edit" and "More actions" don't link to the editor.
The vertical ellipsis 
just gives these options:
How can I add an audio track to my YouTube video?