IFNA Error

When I insert the IFNA formula, it automatically enters a "+" symbol which unables it to function correctly therefore it always comes up as an error. When I delete the "+" symbol, it still appears as an error, and won´t work. In consequence, a  warning appears on my screen telling me to insert an "´" symbol before the IFNA formula, in order to get the function working correctly, I enter it and still doesn´t work.