"+" sign

You are likely in 'Insert Input mode'. See:
https://musescore.org/en/handbook/3/note-input-modes#timewise
which states:
"If, at any time, the total duration of the notes and rests within the measure does not match the time signature, a small + or - sign will be shown above the measure."

Instead, use the default 'Step-time' method of note entry. See:
https://musescore.org/en/handbook/3/note-input-modes#steptime

Also:
https://musescore.org/en/handbook/3/note-input#basic-note-entry