How do I file a state income tax deduction for a qualified 529 plan for Oklahoma?

Unfortunately, Oklahoma can't accept the requested proof for this deduction via an e-filed return.  You'll need to print out Oklahoma's Form 511EF from TurboTax and mail that in with the documentation they request.

For information on the proof requested, please see "Contributions to Oklahoma 529 College Savings Plan and OklahomaDream 529 account(s)" on page 17 of 2019 Oklahoma Resident Individual Income Tax Forms and Instructions. 

So I have the same question.  I paid to file this online and did so, without any warning on the 529 proof.  So I am confused.  Are my taxes files on line and I only need to sign Form 511EF  with the 529 proof and mail to State of Oklahoma and I am all set? 

Yes.  Your taxes are filed online (e-filed), but you must complete Form 511EF and mail it in to the state for the 529 Plan deduction.