Your DNS provider requires an IP address for a DNS A-record in their control panel for root level domains.
Due to the way Heroku works, we are unable to provide you with an IP address to use with an A record for your application. You should consider migrating your DNS to a provider that supports using a CNAME at the root level (read more).
Many DNS providers support using a CNAME value at the root record by implementing their own record type. They will usually refer to these by their own marketing name such as CNAME flattening, ALIAS or ANAME. These different record types all serve the same purpose and will work when adding a custom domain to your application.
Though it is possible to find the IP addresses returned by an application, we do not recommend using these in DNS records as they can and do change frequently.