If the IRS is not able to deposit your refund into your bank account for any reason, the IRS will mail you a check instead.
Sometimes the routing or bank account number is wrong, invalid, missing one digit, or if the name is not exact as the bank account, this can cause rejection of the deposit.
The IRS states a mailed check should take no longer than 3 weeks to arrive.
https://www.irs.gov/newsroom/irs2goapp
