You must have chosen to pay the TurboTax account fees with the federal tax refund. If so, then the option for mailing a check for the refund will not be available.
See this TurboTax support FAQ for removing the pay with refund option and to use a credit/debit card to pay the TurboTax account fee so the refund check option will be available - https://ttlc.intuit.com/community/downgrading/help/how-do-i-remove-pay-with-my-refund-fees-in-turbot...
