I wanted to ask that if people cannot have children, is it possibly a hint or sign from God that they should not be parents?

A person can become more truly childlike by way of having true devotion to Maria Bambina, aka the Child Mary.  Mary is Our Lady of Good Counsel and can advise one with any dilemma in life, especially if he honors her every day by reciting the rosary with care and sincerity.  It is good to say at least the traditional 15 decade of the rosary every day attentively, preferably in Latin but in the vernacular if one does not yet know the Latin.

That's like saying that if they cannot properly digest food, its because God wants them to starve to death. We live in a natural world, and natural imperfections occur for natural reasons in natural organisms, including humans. Nothing supernatural about it.

Usually not.  Plenty of horrible people have children.

No. God would not be the one to “blame” in this situation. It’s just an unfortunate effect of imperfection. Some people can physically have children, some cannot. 

No -----------------

No, and there are plenty of examples of couples who had children who shouldn't have been parents.  Fortunately for the child, sometimes even those parents recognize that and give the child up for adoption to a couple who become great parents to the child, even though the wife is barren.

Funny how God never seems to tell the abusive parents who rape their children, beat their toddlers to death or pimp their daughters out for drugs  that they should not be parents.

No. Using that same logic, having a miscarriage is a sign from God that he is in favor of abortions since He Himself performs them.

No, primarily because no gods exist.

Plus, we need people to adopt orphans.

No