How to disable two-sided printing

How can I disable two-sided printing? Although the Print Layout settings show that two-sided printing is Off (in fact, the setting is gray and can't be changed), all documents always print two-sided. Often the print quality of the reverse side is poor and smudged. It's driving me nuts that no matter what I do, I can't change the outcome. I'm running Big Sur, OS 11.2.3, but this has been happening for awhile. I'm using a Canon Image Class D480 printer. I hope someone can help.