AirPort Utility will tell you what the issue might be if you.......
Open AirPort Utility
Click on the AirPort Extreme
Look to see if an "Update" button is present in the smaller window that appears
If an Update button appears, click on it to update the firmware on the AirPort Extreme. The example below shows an AirPort Express, but your will see your device on your Mac
If an "Update" 'button does not appear........look for the Status setting and click on the small amber dot there. Another window will open up with more information about why the AirPort Extreme is complaining.