When you enter a formula incorrectly into a cell , Excel displays an error message. Formulas can be entered incorrectly for a variety of reasons. Typing mistakes are usually the biggest cause of formula errors. If you mistype an operator or function name , Excel returns an error instead of the desired result.
The next biggest reason that formulas return error values is improper syntax. Syntax is everything when you're entering formulas; your formula must conform exactly to Excel's format. For example, if your formula contains a blank space, the formula errors out.
Excel often provides information about the errors it finds in formulas. With Excel's help, you can usually fix the problem. If the formula looks okay but still isn't producing the results you expect, check it over. If you inadvertently reference the wrong cell or use the wrong function, such as SUM instead of AVERAGE, your answer will not be correct.