Some DNS providers do not allow end users to add required DMARC, SPF or DKIM records within their UI. We have heard from our customers this is the case with AT&T.
Connecting with AT&T
AT&T DNS Support can add the required records for you.
You can file a ticket directly or email AT&T DNS Changes at firstname.lastname@example.org
You can also call 888-787-4288 and select prompt #3 to speak with the DNS Team.
You will be required to provide your account ID and possibly your WAN IP to the support staff. This should be available in your AT&T account details.