This article describes the formula syntax and usage of the NAfunction in Microsoft Excel.
Returns the error value #N/A. #N/A is the error value that means "no value is available." Use NA to mark empty cells. By entering #N/A in cells where you are missing information, you can avoid the problem of unintentionally including empty cells in your calculations. (When a formula refers to a cell containing #N/A, the formula returns the #N/A error value.)
The NA function syntax has no arguments.
You must include the empty parentheses with the function name. Otherwise, Microsoft Excel will not recognize it as a function.
You can also type the value #N/A directly into a cell. The NA function is provided for compatibility with other spreadsheet programs.