how do I add a new voice on a new line

See https://musescore.org/en/handbook/3/note-input and https://musescore.org/en/handbook/3/voices
Or do you want to add anozther staff or instrument( if so press I and add a staff to the existing instrument or add another instrument in that dialog

And update from 3.2.3 to 3.6.2 ;-)