First off, welcome to PB fellow Miserian!

The Wildlife Code of Missouri is the gold standard for what is/isn't permitted in Missouri.
Wildlife Code of Missouri

A search of the current document results in zero hits for "tilapia".

Similarly, there is no mention of "tilapia" on the MO Dept of Conservation website.
MO Dept of Conservation

That said, consider the fact that the Missouri Wildlife Code is a "permissive" code, meaning that if it does not specifically
state that a given thing is permitted, it is NOT permitted. Pay particular attention to 3 CSR 10-9.110 General Prohibition; Applications.
The fact that tilapia is NOT on the list of species permitted for aquaculture would suggest that tilapia are NOT permitted.

And that said, pay particular attention to 3 CSR 10-4.117 Prohibited Species.
Tilapia is NOT a species that is specifically prohibited.

Considered together, those two sections of the code may lead one to believe that tilapia are in a gray area, as evidenced
by the fact that several fish sellers can be found who are perfectly willing to sell tilapia into Missouri.

Additional information on aquaculture in Missouri can be found here.
Missouri Aquaculture Association

I am not an attorney, so DO NOT interpret my comments as any sort of legal advice.

I can state this with certainty: the Wildlife Code of Missouri as published on the Secretary of State website IS the law, and anyone
considering the import of a species not specifically permitted by the code should obtain competent legal advice before
importing non-native fish or wildlife to the state.

Hope that clears it up for you.