Canoscan 5600f and Catalina
The Canoscan 5600f is a lovely scanner. Canon has elected not to provide 64 bit driver that works with Catalina. In theory, Apple has solved this problem with two approaches. The first is Image Capture. Note this image from Image Capture's Help menu.
The second approach is to go to System Preferences/Printers and Scanners/, select the 5600f under scanners, and then click on Open Scanner.
Both Image Capture and System Preferences occasionally work and both occasionally don't work. I'll leave it to you to discover the many things you can try to make it work. Closing and re-opening the software seems to work best, although clicking on Scan is a close 2nd.
I've been working to find a solution to this problem since October, 2019 via Apple. I've found that 1st level support has no training win this area. There also seems to be no Apple group responsible for Image Capture.
Canon offers zero help.
Hamrick offers Vuescan for a reasonable price. Their support has no manners and doesn't listen.
I would appreciate hearing any experiences that you have had with this problem. If you have any free time, contact Apple support, ask for a senior support person with printer/scanner knowledge and see if you can find somebody that can help.
iMac 21.5", macOS 10.15