Mo. Rev. Stat. § 67.1561

No lawsuit to set aside a district established, or a special assessment or a tax levied under sections 67.1401 to 67.1571 or to otherwise question the validity of the proceedings related thereto shall be brought after the expiration of ninety days from the effective date of the ordinance establishing such district in question or the election establishing a district pursuant to section 67.1422 or the effective date of the resolution levying such special assessment or tax in question or the effective date of a merger of two districts under section 67.1485.