Lacerte diagnostic says: "The Relationship selected is invalid for electronic filing purposes. The following are valid relationships: Son, Daughter, Foster Child, Grandchild, Grandparent, Parent, Brother, Sister, Aunt, Uncle, Nephew, Niece, Stepchild, Half Brother, Half Sister, Stepbrother, Stepsister, None or Other."

So it's that the IRS only recognizes nonbinary people for some categories of relative.