@hudspethofs We need more information to be able to better assist you. Where is the email you received from? And provide more details about the request. What state are you filing taxes? I found some information below but am not sure it is relevant to your situation.
The only information I can find on the D400 form is related to North Carolina state taxes. The Form D-400, Individual Income Tax Return Main Form is the tax return you file for the state of North Carolina. It is estimated to be available for filing on 02/17/2022. And that form is not available yet.
If you still need further assistance, please respond in this thread.
1) It isn't in your own documents...The D-400 is the NC form that TTX has to fill out for you to send to NC when you complete filling in all your information/income/deductions/credits etc.
2) Only the Printed final form D-400 is available i.e. only if you decide to print out your NC tax return, and file it yourself by mail....the notice was extremely poorly worded about that.
The current list of state forms for E-filing indicates you cannot E-file your NC tax return until 24 Feb (or later).
_________________________________
IF you want to see what the D-400 looks like, you can get a copy here (instructions are the D-401):
Individual Income Tax Forms & Instructions | NCDOR
The email came to me from TurboTax. While filling out my state (Nc) tax return it told me I was unable to process my taxes until certain forms were available. So now I have received an email saying form D400 is available. My state taxes say pending but my federal says approved. It took 1 day for federal and has been over a week for state and still showing pending.
Many of the North Carolina state forms are not available yet for e-filing. You can monitor the status of the forms needed for your North Carolina tax return in the below link. If the forms become available sooner than the currently published date, the article will be updated to reflect the new availability date.