I finally got it working (with Sonoma 14.7.1, but I doubt that this made any difference).
1. See that the slider on the underside of the scanner is moved to the ON (C-like open circle)
position. The brief instructions that come with the scanner have a diagram (in item 2) that is
intended to convey this information, but the text does not mention it. (The off position fixes
moving parts in place for transit.)
2. Click on the apple on the top left of the screen, then click on 'System Settings', then
scroll down to and click on 'Printers & Scanners', then click on Add Printer, Scanner or
Fax' and (assuming it is connected) you should be able to add the Canon LiDE 400.
This should surely be mentioned in the brief instructions that come with the scanner!
3. The scanner worked fine once I had restarted my laptop and restarted the
Canon IJ Scan Utility Lite app.
