The Valimail DMARC and DKIM configuration are standard DNS delegations and are supported by any DNS server. Some DNS providers, typically those focussed on small business and non technical customers choose to limit what customers can configure in DNS. They do this to make things easier for their customers and to reduce their support burden. Unfortunately, one of the things that sometimes get excluded from the providers' portals are the ability to delegate subdomains.
If the customer is using a DNS provider that has this restriction, the customer has two choices: