You are unable to change your primary domain name since the option Make Primary is missing.
- Admin console
- Domain Management
- Open the Google Admin console.
- Go to Domains > Manage domains.
- Next to the domain alias domain in question, click Remove.
- Click Remove alias domain.
- Re-add the domain as a secondary domain.
- Next to the new secondary domain, click Make Primary.
- Click Change Primary Domain.
In order for a domain to become a primary domain it must be added as a secondary domain, user alias domains cannot be converted into primary domains.