This means that the IRS has not finished processing your tax return as of yet. With the new tax laws and updates, the IRS is taking every precaution to ensure each tax return is processed correctly and fully. There's no way to know when the IRS will update. As your return moves through the system, the status of it will be updated accordingly. Continue checking the Where’s My Federal Refund? tool for updates. Updates are made daily, usually overnight.
If after 21 days of being accepted, you have not received an update via the Where's My Refund tool, you can follow the instructions in the following FAQ to contact the IRS for further assistance.
https://ttlc.intuit.com/community/refund-status/help/what-if-my-irs-refund-is-taking-longer-than-21-days/00/26080