Using Numbers, unable to type in header

I've been trying to change the year that is put into the header on a Number's spreadsheet. I can place the cursor at the end of say, "2020", and hit the delete button, and the year is removed. But when typing a number to replace it (or anything typed), nothing appears in the header. I've gotten around this by copying the text "2021" and pasting it in the header. For whatever reason, the header is not accepting anything typed. (I've restarted my machine . . . still doesn't work.)