How to fix 'failed to open connection to device (-21345)' error for Canon MG4200 series scanner on macOS Sequoia?

Final: There are actually several threads describing the same issue you seem to have. The first thing I would suggest you to do is looking for a firmware update for your MFP. Also, solutions people presented were these:

possible solutions:

1. antonlinus suggested this: while other posts suggest the solution is resetting the printer system under system preferences this was not a complete working solution for me. As a second step i needed to go to my printers websetup page (usually the printers IP adress) and deactivate IPv6 completely. only after deactivating and then resetting the printer system in macOS i was able to regain access to my scanner completely.
2. Remove your MFP overall, reboot and add it again. ccbnyc reported success with this.
3. make sure you have current drivers installed
4. try to use AirPrint scan

some of the threads still don’t seem to have their problems solved, if nothing of this works for you, may I suggest you to contact Apple. Contact - Official Apple Support

Go to  > System Settings > Privacy & Security > Local Network, and make sure everything related to your Canon printer is turned on.

There are actually several threads describing the same issue you seem to have. The first thing I would suggest you to do is looking for a firmware update for your MFP. Also, solutions people presented were these:

possible solutions:

1. antonlinus suggested this: while other posts suggest the solution is resetting the printer system under system preferences this was not a complete working solution for me. As a second step i needed to go to my printers websetup page (usually the printers IP adress) and deactivate IPv6 completely. only after deactivating and then resetting the printer system in macOS i was able to regain access to my scanner completely.
2. Remove your MFP overall, reboot and add it again. ccbnyc reported success with this.
3. w.i.p.

This solved the problem for me! The slider of CANON IJ scan utility was turned off. Thanks.

nsw1654 wrote:

Thanks for this. Still no luck Scanner seen but no connection.

You're welcome.

Give the trial version of VueScan from > https://www.hamrick.com/ a try.

+1 In addition to Mac Jim ID's advice, test to see if Image Capture works.

see > Scan images in Image Capture on Mac - Apple Support

and > Solve scanning problems on Mac - Apple Support