The 'domain verified by another business' issue must be solved as soon as possible, as it will prevent you from running ads on Facebook.
You can identify this issue by either:
Clicking on the "Fix" button in the red error banner.
By navigating to Settings => Facebook integration or clicking here.
Resolving the issue:
Find out which business has ownership over your domain.
This can be an old business manager, an ad agency you worked with, and more.
Enter that business manager.
Make sure you are in the business that has ownership over your domain.
On the left-hand menu, click Brand Safety => Domains.
Select the domain you want to share.
Click Assign partners, and enter the business ID you wish to share access with.
Select Manage Domain, and click save.
Once you have completed these steps, navigate to your Facebook integration screen and click "Recheck Status".
The domain is verified by another business issue and should no longer appear.
If it is still there after following these steps, please contact support.