Can a couple who cannot have children still have a valid marriage?

Yes, it can; the church says they can. They can always adopt children, and there is more than one way than just the natural one flesh of the parents. Besides that, they do not even know for certain whether they are infertile. Sarah thought she would be barren her whole life and was not; Hannah prayed night and day because she was infertile, and God opened her womb, and you can read that whole story in 1 Samuel chapter 1. There have been many people who thought they were infertile and were not. So yes, a couple that cannot have children can have a valid marriage.