This MSG kept on displaying on the LCD along with the red flashing light and a sound when a print job was given to MF6550. On top of that every time the cassette was arraged and placed correctly or left deck was checked for blocked papers and closed the same page was printed everytime. The manual of MF6550 had no solution to such scenario.
Few googl'ing gave a link/pdf on "How to fix the check paper size error msg for MF5600/MF5700" series laser multifunctional.
The panels were different but it did the work.
Canon ImageClass MF6550

Carrying out the following step didn't solve the problem (as indicated by the err msg)

Check the paper settings on the Printer
For Paper Cassette the default settings are <Plain paper> and <LTR>. When you place other types of paper, the steps to be followed are
  •  Press [Additional Functions] (button 2)
  •  Confirm that <Common settings> is displayed, then press [Ok] (button 4)
  •  Press [-] or [+] to select <select paper type>, then press [OK] (button 3,4,5)
  •  Confirm that <Cassette 1> is displayed, then press [OK]
  •  Press [-] or [+] to select the paper type, then press [OK]
  •  Confirm that <Reg. Paper size> is displayed, then press [OK]
  •  Confirm that <Cassette 1> is displayed, then press [OK]
  •  Press [-] or [+] to select the paper size, then press [OK]
  •  Press [STOP] to return to standby mode
The below given step did the work
Apart from removing the printjob from desktop

Removing jobs from the print Queue (logging what could be recalled)
  •  Press [System Monitor] (Button 6)
  •  Press  [(-)] or [(+)] to select <Print Status> (Button 5 or 3)
  •  Press OK (Button 4)
  •  Press OK to delete the Print Job
ref:- Manual of the MF6550 + checkpapererror.pdf of MF5700