Why will iMac recognize Canon all in one printer but not scanner?

You can also use the Image Capture app that is pre-installed on your Mac and use IC to scan. If you are not familiar using Image Capture for scanning please refer to: Scan images in Image Capture on Mac - Apple Support

This is because they are two different functions. A printer usually needs a print driver in order to print. The scanning function requires scanner software. Very likely you recently upgraded your computer's operating system and the scanner no longer works. Now you need to upgrade its software to a version that works with your OS. That will be found on the Cannon web support page, if one has been released.

It is important to keep in mind that printing and scanning are two distinct functions. Unless and until your Canon software is updated, the scan function will not resume working. A noted, you may be able to scan using Apple's Image Capture app, but it is not a full featured scanner software. you can also install VueScan software and it should work nicely. It is a full featured scanner software.

Please read here and restate your issue: Writing an effective Apple Support Commun… - Apple Community

What model? My Canon TS8320 all in one scans and prints as long as all the Canon Software is installed.