The scanner lock on the MP980 is on left hand side of the scanner unit. It's closer to the hinge of the document cover and almost centered between the scanner glass and the edge of the printer. If you lifted the entire scanner unit, you won't see the switch. You should be able to push the switch towards the open lock symbol with your finger or thumb. The Getting Started guide has a pretty good picture of where the switch is in Step One. The guide is available on the product page, http://www.usa.canon.com/cusa/support/consumer/printers_multifunction/pixma_mp_series/pixma_mp980#BrochuresAndManuals
, in the Brochures & Manuals section. The current link to the Getting Started Guide is http://gdlp01.c-wss.com/gds/2/0300001552/01/MP980_GS_EN-US_V3.pdf
. After moving the switch, turn off the all-in-one. Then unplug the power cord to clear the error message from the printer's memory. After a few minutes, plug the power cord back in and turn on the printer.
If the error persists and the scanner is unlocked, the scanner mechanism may be damaged. If that's the case, your MP980 needs service. If the unit is under warranty, contact Canon. (If it isn't under warranty, you'll need to decide if the unit is worth the shipping and handling fees and the minimum $50 or so charge just to determine the problem at a service center. (That fee will be applied to the cost of fixing the printer, if you decide to have the technician continue to work on the unit.)
I hope this helps.