If for any reason you can not have children. For example
Doctors told you that you can not have children for some medical reason.
Every thing is ok but you just can not get pregnant.
You been married for long time and you are not getting pregnant

In any case what so ever please do THE KING OF ALL WAZIFAS NO 2 from the website.
Ask Allah for a child and you will get it. Be patient with it and keep on doing it until you are pregnant.