"Ready to mail" is related to your chosen filing method, not your refund method.
Direct deposit is the method the IRS is going to give you your tax refund. But you need to send the IRS your tax return before they can send you your refund (via direct deposit).
There are two filing methods in TurboTax: e-filing and mail. If you are seeing "Ready to mail", it means you have chosen to file your taxes in the mail. You should print your tax return and send it to the IRS. The mailing address and other instructions will be included when you print your tax return.
