Is there such thing as not being able to have kids on VF?

No, there isn't. As LadyCFII states in this post, the key to having children is persistence.

After I found out that when they argued about kids I just had to be persistent, I had no problems. I usually waited until the last child was 3-5 years old before trying again though.

If you are persistent they will eventually try to have
a baby! Some couples have more trouble having children though. Using the baby boost can help! But if you give them the baby boost more then once it has the opposite affect! Also if you really want them to have a lot of children try to marry young because after a certain age it gets harder!