4/4 time measure will not except 4 beats but only three.

You have inadvertently changed the “Actual Duration” of the measure. That’s what the little gray plus and minus signs mean.

Right-click on the measure in question. Do not right-click on a note, rest, or text; right-click on empty space within the measure. Click on Measure properties… (or possibly Bar properties…) at the bottom of the context menu. On the Measure properties (or Bar properties) dialog, change the Actual Duration to “4/4“.

For further details, see these two sections of the Handbook:

musescore.org/en/handbook/4/pickup-and-non-metered-measures
musescore.org/en/handbook/4/measure-properties