21 days is not a promise---it was an estimate. Some returns take longer to be processed than others.
https://ttlc.intuit.com/questions/1901548-why-do-some-refunds-take-longer-than-others
https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
https://ttlc.intuit.com/questions/2840013-does-accepted-mean-my-refund-is-approved
https://ttlc.intuit.com/questions/2724106-my-return-is-accepted-but-still-not-approved-is-there-a-pr...
