Is it true that uncircumcised men can't have a child? Is there a scientific explanation? Is this a sin being uncircumcised?
It is a falsehood that you can easily prove to yourself. In the ancient world, around the time of Jesus, the Jews were known as "the circumcised" and anyone who was not a Jew was known as "the uncircumcised." Do you need to guess why? I thought not. If it were true that uncircumcised men could not father babies, then the Gentiles would have died out thousands of years ago.
Circumcision is a cosmetic operation; that is, it changes a person's looks but it provides no direct benefits. Under the Old Testament, God made a covenant with Abraham and his descendants. As a witness to that covenant (proof that the person is under the covenant), God told Abraham to circumcise all the males. "This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised; and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. He who is eight days old among you shall be circumcised, every male child in your generations, he who is born in your house or bought with money from any foreigner who is not your descendant. He who is born in your house and he who is bought with your money must be circumcised, and My covenant shall be in your flesh for an everlasting covenant. And the uncircumcised male child, who is not circumcised in the flesh of his foreskin, that person shall be cut off from his people; he has broken My covenant" (Genesis 17:10-14).
The witness to the covenant only makes sense while the covenant exists. Without the covenant, circumcision would make as much sense as someone wearing a wedding ring without taking wedding vows.
The New Testament (covenant) replaced the Old Testament and along with that change came a change in the terms of the covenant. Among these changes circumcision is not a witness to the new covenant. "Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage. Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace" (Galatians 5:1-4). Therefore, it is the opposite of what you asked. If a person gets circumcised for religious reasons, he has committed a sin. He is refusing to listen to God who said the law has changed and by doing so has separated himself from Christ.
Now some cultures practice circumcision, but they do it to babies like the Jews. Such is done in the United States where about 50% of the children are circumcised. In other cultures it is a rite of passage, such as in the Philippines, were it done sometime around puberty. Many times people learn about the truth of Christ long after they had been circumcised. What should a person do? "Was anyone called while circumcised? Let him not become uncircumcised. Was anyone called while uncircumcised? Let him not be circumcised. Circumcision is nothing and uncircumcision is nothing, but keeping the commandments of God is what matters. Let each one remain in the same calling in which he was called" (I Corinthians 7:18-20).