Which courses should I take online and on which platform to learn how to design wall art that can be sold by printing it on canvas via print on demand platforms?

Sounds as though you have no training and or Creative Abilities. Due to AI, there are thousands of people who now consider themselves Artists. To me this is laughable. Due to all of these overnight Artistic Icons, the internet marketing of Art has skyrocketed with artificial images yet on a positive side, some have made a bit of money on the LOW END Market. Also, prints on Canvas saw its heyday during the 1960’s. You would be better off silk screening underware. You should also realize that there are people who appreciate Fine Art and many are buyers and collectors of legitimate Art and the incredible humans who create this. I have seen some 3 dimensional works created by AI that have achieved a niche level of success as these are also very low price. You have asked the wrong person for answers to your question. I have worked in the world of fine art most of my life. Best of luck!