Form 8879 is used to authorize a paid tax preparer to e-file your tax return for you. It is not used when you file your tax return yourself with TurboTax. You don't have to authorize yourself to file your own tax return. If the bank is asking you for Form 8879, either they think you paid someone to do your tax return, or they don't understand how tax returns are filed.
You could try explaining to the person who's asking for it that you filed by yourself, and show them the Electronic Postmark, which shows that you filed your tax return. If that doesn't work, try asking to speak to a supervisor or other higher-level person who knows more about tax return filing. If they still insist on getting Form 8879, consider switching to a competent bank.
