You will want to carefully review the family the selected person is attached to showing the "no spouse entered".
This "person" is often created by FTM when a child is attached to a single person as a parent. FTM has to have a couple as a set of parents - it can't handle single persons as parents.
If the screen at Attach/Detach > Detach show children in the house, you will want to exit that screen and go back and look at the children. In most cases, you will want to actually give the "no spouse entered" a name, such as "wife" , unknown wife", "first wife", "second wife", or whatever and not detach anybody.
In other cases, you may have added the child to a parent before you recorded their marriage. As a result, you may already have the mother / father as another wife / husband, and will need to detach the child from the wrong set of parents and attach the child to the correct parents.