Canon NP6551 Operator's Manual page 209

Message/Cause
The Side Paper Deck (Option) is out of order.
The feeder is out of order.
The RDF is out of order.
Two-sided copies cannot be
made with this mode.
Overlay copies cannot be
made with this paper size.
The copy paper size that you selected
cannot be used to make copies in the
set mode (e.g., STMTR in the
Two-sided mode or STMTR in the
Overlay mode.)
Place 2 or more originals.
Because 1→2-sided copying is set as
the standard mode, copying is not
possible if only one sheet of paper has
been placed in the RDF.
Place the 2nd original and press
the Start key again.
Copying of one side is completed, and
the copy paper is stored in the
duplexing unit.
Remedy
Contact your service representative.
(You can use paper drawers 1, 2, 3, 4.)
Contact your service representative.
(You can use the platen glass to perform
copying.)
Select a different paper size, or cancel
the mode and program the copy settings
again.
Either place two or more originals in the
RDF or select the One-sided mode
before copying.
Replace the originals and resume
copying.
• Load the next original and press the
Start key.
• If this message is displayed after the
last original has been copied, remove
the paper from the duplexing unit.
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