The 21 days to receive a federal tax refund after the IRS has accepted the return is only an IRS target, it is not a guarantee. There are many reasons why the tax refund could be delayed.
If accepted by the IRS use the federal tax refund website to check the refund status - https://www.irs.gov/refunds
After the tax return has been Accepted by the IRS (meaning only that they received the return) it will be in the Processing mode until the tax refund has been Approved and then an Issue Date will be available on the IRS website.
See this IRS website for federal tax refund FAQ's - https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
Once a tax return has been Accepted by the IRS or a State, TurboTax receives no further information concerning the tax return or the status of any tax refund.