Unfortunately,
Oklahoma can't accept the requested proof via an eFiled 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 2018 Oklahoma Resident
Individual Income Tax Forms and Instructions.
You can also see
Form 511EF and its instructions, including mailing address, here.
