a) there is no need for E12/C12 to be within brackets
b) the "" marks would not normally have a space between them if you don't want anything to be inserted in the cell. I don't get an error message with that formula.
"initially in F12 I had “=E12/C12”, which worked when C12 was greater than 0, but when C12 was 0 the error message appeared!"
An error would appear because dividing by zero gives infinity, which cannot be calculated.