Mo. Rev. Stat. § 327.461: Contract with unlicensed architect, professional engineer, professional land surveyor, or professional landscape architect unenforceable by them.

Every contract for architectural or engineering or land surveying or landscape architectural services entered into by any person who is not an architect or professional engineer or professional land surveyor or professional landscape architect, as the case may be, and who is not exempt from the provisions of this chapter, shall be unenforceable by the unlicensed or unauthorized person, whether in contract, quantum meruit or other legal theory, regardless of whether a benefit has been conferred.