I have the TS6300 and am having this same problem reguarly, fromting from my Macbook Pro OSX Catalina 10.15.7. The following example is from MS Word, but this happens in many situations.

I have regulation letter-size paper loaded in the cassette. There is nothing in the rear feed, and it is closed (this picture is for illustration only, when I try to print the main cassette is properly inserted, of course):

From the print dialog I choose Main Tray (the only other option being Rear Tray). I have tried specifying the media typel it makes no difference, nor does the quality setting:

I hit print. The printer immediately responds that there is a problem with the paper loaded in the rear tray... which is not the tray I want to deal with at all:

This is becoming more and more of a nuisance, and I also expect the driver or firmware are at fault.

Is there any expected fix for this any time soon? If the answer ends up being "You just need to buy a newer printer" (this one being only a year old or so), that will be unacceptable to the point that it may cause me to end my relationship with Canon. 

Please tell me there is a reliable and easy solution for this.

Thank you.