The error message "Paper Size Mismatch" or "Wrong paper size" is displayed when the paper size setting does not match the size of paper in the tray.

Or, you did not set the paper guides in the tray to the indicators for the paper size you are using.
 

If you are printing from your computer, make sure the paper size you selected in the printer driver or in the application you use to print matches the size of paper in the tray.