"This student is a parent or guardian in a family: remove them from that role first."

Where: Edit an adult's profile, checking Minor student.

Why: this person is a parent or guardian in a family, and a minor cannot be one.

What to do: open the profile of the child they are a parent of, tap ⋮ next to them in Parents and guardians, then Remove; then come back and check Minor student. Nothing was changed in the meantime.