Mike Myers is this year’s only independent candidate for Governor of South Dakota. According to this story, his Lieutenant Governor running mate must drop out of the race for health reasons, and the Secretary of State says Myers can’t replace him.
However, federal courts in Virginia and Florida have ruled that if states permit qualified parties to substitute, they must let unqualified parties or independent candidates enjoy the same substitution ability. The Florida case is Anderson v Firestone, 499 F Supp 1027 (n.d. Fla 1980); the Virginia case is El-Amin v State Board of Elections, 721 F Supp 770 (e.d. Va. 1989).