This issue may occur even when the phone number is verified and correctly entered. To resolve it:
1) Navigate to Business Settings → Business Info, remove the current phone number, save changes, then re-enter it using the proper international format (e.g., +1XXXXXXXXXX).
2) Use a desktop browser in incognito mode; some users report issues when using mobile or outdated browsers.
3) After saving the number, log out and log back in to refresh account validation.
4) If the warning still shows up, try creating a new campaign instead of duplicating an existing one. That has helped resolve similar bugs.
5) If none of these steps work, reaching out to Meta support is recommended. The error appears to be a temporary validation bug affecting multiple accounts.