Your registration status changed to "incomplete" because the IRS identified an issue with your registration. Several specific issues could have caused this change:
Additionally, if the registration was initiated or submitted but not completed within 28 days, the status would change to "incomplete." This applies both to registrations that were in the "initiated" or "registration submitted" status and those that were in the "approved" status but not completed within the specified timeframe.
To correct this, you need to log into your FATCA account, update the necessary information, and resubmit the registration. The system allows you to edit and complete the registration from the beginning of Part 1 or Part 2 of the online registration. Once the registration is updated and resubmitted, the status can change back to "registration submitted" or "approved."
Sources:
Publication 5118 (07/2021)
Publication 5884 (11/2023)