What are the different domain statuses?

Not Verified

Your domain will be under "Not Verified" state only when the either of the SPF or DKIM records are not updated correctly in your domain's DNS.

How to get rid of "Not Verified" state?
Click on View Settings under Sending Domains dashboard and make sure you have added the required SPF and DKIM records into your domain's DNS. Once the records are updated at your end, click on "Check DNS Records Now" button under settings to refresh the domain's status.