Samsung has never allowed the heating elements of its ovens/ranges to be turned on just with the app. That’s pretty common in the industry: it’s considered a safety feature. (I myself am quadriparetic, use a wheelchair and have limited hand function, so I follow this device category pretty closely.)
I don’t know about yours, but some models do have a physical button on the stove that you can press, which will then allow you to use some additional controls via the app. So you might check the manual to see if you have that option.
Here’s a long discussion thread, but the short answer is they just don’t offer that feature.