21 days is not a promise.
The IRS has been severely backlogged for a long time. Many people are getting this message. TurboTax does not get any information after you file your tax return so no one at TT knows a specific reason why you are getting that message. If the IRS wants any other information from you they will let you know by letter. Meanwhile all you can do is wait patiently and watch the IRS site. The IRS is your only source of information.