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The cover is signaled as open, either because it actually is (at one corner, slightly, because something e.g. a paper bit, is stuck in it in one place, etc.), or else because the cover sensor has failed and is indicating falsely that it is open... or perhaps because of a circuit board malfunction producing a false cover-open indication.
Open the cover (top) on the printer... examine the rim where the cover fits down, for any extraneous debris or paper bits, etc., and remove any such obstruction. Then close it again, firmly, examining it on all sides for any looseness of fit.
Then press the CANCEL button or the STOP button, to try to clear the error if it still shows.
After which you may assume your printer needs servicing to clear the error and to restore operating condition. How old is the printer? if it is older than 5 years in-use time, you may consider simply replacing it, both as cheaper than having it service-repaired out of warranty, or as more effective in the long run.
(If within 1 year old, having it service-repaired would be best - repairs would be free of charge under Canon's warranty... let me know if this is the case, so that I may advise you how to proceed.)
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