Missing Certificate

Some users  are reporting seeing the message:

"This custom domain is missing its certificate. Please delete and recreate the assignment"

If the Google Site is actually working at the custom URL, this message can be ignored.

Background:
There is an ongoing change in the domain ownership verification called Multi party issuance corroboration (MPIC) which aims to improve web security by checking DNS records from multiple geographical locations.  In cases where the DNS settings do not resolve globally consistently, this may cause apparent failure in domain verification.
Customers need to work with their domain registrar to ensure that their DNS settings resolve globally.
They can also use a DNS validation propagation checker of their choice to verify that all relevant DNS records resolve correctly and consistently across the globe e.g https://www.whatsmydns.net/ 

The issue might also be caused due to a DNS conflict arising from having both CNAME and A records configured for the same domain hostname www.mydomainname.com.  Remove the A records linked to www.mydomainname.com (if any) and only retain the CNAME record pointing to "ghs.googlehosted.com." and then retry the domain mapping.