21 days is just an estimated processing time but not a guarantee. The IRS is taking much longer this year.
If your return was Accepted or mailed then check IRS Where's my refund
https://www.irs.gov/refunds/
When will you get your refund?
https://ttlc.intuit.com/community/refund-status/help/when-will-i-get-my-irs-tax-refund/01/25589
IRS - Questions about Refunds
https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
