Hello Michelle,
On this model, the kitchen timer and the cooking function are designed to operate separately rather than simultaneously. When the timer is active, the keypad prioritizes adjusting or interacting with that timer, which is why entering numbers changes the remaining time instead of starting a cooking cycle.
To begin heating food, the timer would need to be cleared or finished first, after which you can enter your cook time and start the microwave normally.
Glad to help clarify this.
Nicole
Samsung Care