The Excel NA function returns the Excel #N/A error. This error message denotes 'value not available' and is produced when an Excel Formula is unable to find a value that it needs.
The function takes no arguments and therefore, the format is simply:
Note that you can also get the same result by simply typing #N/A directly into a worksheet cell.
For more details on the Excel #N/A error, and other Excel error types, go to the Excel Formula Errors page.
Also, further information on the Excel NA function can be found on the Microsoft Office website