They tried learning after a couple of minutes but got frustrated and says they do not want to try it anymore. Should I keep pushing them or just let it be?
yes - gross motor skills are important to healthy muscle development, besides using two feet in opposite directions helps with brain functions also. bike riding is an important part of childhood, mom says since she couldn't see well as a child she never learned to ride and the children made fun of her.
Don't push. They'll want to on their own, eventually. And if not, it's not as important as learning how to swim. Now that's what I got insistent on them learning.
They should probably learn eventually. I know several adults that don't know how, and are kind of embarrassed about it. Maybe try training wheels. They helped me a lot when I was little.