Ok, this was the issue for me. After "resetting" Win 10 ver 2004 ( that's another convoluted story), your apps are deleted and they all need to be reinstalled including the Cannon Printer drivers. Once the "3" Cannon files are installed (driver, patch and USB language fix) you have the "default" settings. One of the settings was 2-sided, which was apparently OFF. I didn't notice this and when the error message said Cassette Tray A4, I changed the tray to 1. CASSETTE from 2. MP TRAY. - Wrong. I should have just changed the 2-sided setting. So I wasn't getting two sided because a) 2-sided was OFF and b) I selected the wrong cassette. I found it difficult to find the 2-sided settings. Go to ADD Functions. Cycle through FAX settings and PRINTER setting and the third Item is 2-sided copying. Use the + / - to select ON or OFF.
Summary
a) finding the 2-sided setting from ADD Functions and select the 2-sided button to be sure it is ON.
b) change the Cassette to Multi-Purpose (MP) TRAY.
Good luck.
