For configuring ServiceNow custom URL one of the basic requirements is to create a CNAME on the DNS that points to the instance.
Creating the CNAME is failing with "Cannot create CNAME record at the APEX domain"
CNAME cannot be created at the root/apex of a domain, for example: myservicenow.com
As seen in the documentation the customurl should be of the format servicenow.acme.com and not acme.com, the fully qualified domain name (FQDN) of the custom URL.
Note: For example, servicenow.acme.com is the FQDN of the https://servicenow.acme.com URL.