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 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 refer to these by their own marketing name, CNAME flattening, ALIAS and ANAME but they all serve the same purpose.
Do not lookup the IP addresses returned by application - these can and do change frequently.