Need to be able to use dashes or 0s in cells; getting error from excel

I have a spreadsheet I'm working on where I need to be able to enter 0% as a value sometimes, and other times I need to be able to enter a dash as a way to indicated n/a. I can't figure out how to even enter a dash without getting a "don't leave cell empty, enter a 0" warning from excel. Is there a way to format this that will let me do either? I've tried playing with conditional formatting and custom cell formats but can't figure it out.

Also does anyone know where that "don't leave this cell empty" error might be coming from? It will let me do zeroes and dashes in other cells, but this spreadsheet was given to me pre-made and I think they must have changed some setting on these cells that I don't know how to undo. Tried googling and got frustrated.