"Still being processed" is displayed after 21 days for anything that requires further review by the IRS. There are a number of things that can cause it. In some cases it may be to ID verify, while in other cases it may be due to tax credit issues, such as those related to EITC, ACTC, stimulus payments, etc., or the return may have some missing information or a missing form/schedule, calculation errors, etc. The IRS often fixes what it can, and if more info is needed or any action is taken, they should send a letter of explanation.
Currently the IRS is about 16 million returns behind for any 2020 returns that require a manual review/processing.
