Suspect error in manual p.60 under "How to enter notes in different voices" point 3
MuseScore Version: 18.104.22.1688021803, Revision: 322f34,
Windows: Microsoft Windows 10 Pro, Version: 10.0.19043 Build 19043
After 'Click on the "Voice 2" button . . . and before Enter all the lower voice notes (down-stem).'
This makes the note selected a voice 2 note but simultaneously a voice 1 pause was created.
Select the (blue) pause and select Voice 2, this switches the voicing of the originally selected note back as a voice 1 note and the pause is now marked as a (green) voice 2 pause.
and then use the original text: 'Enter all the lower voice notes (down-stem).'
I'm sure most beginners like me stumbled on this one originally but eventually figured it out but . . . ;o)