Customers would like the ability to assign their root domain to their PowerApps portals - www is so old school!
Typically this would require an A record, which would not be ideal as the IP Addresses will vary within Microsoft's environment.
DNS services such as CloudFlare (and others) have a solution called CNAME flattening, where they serve a dynamic A record to the standard DNS resolution, but in the background it is configured to a CNAME such as xyzcustomer.powerappsportals.com. This works perfectly with other sites I have used it on.
Unfortunately PowerApps Portals custom domain config will only allow you to configure this with a direct CNAME record, so when you try to configure the root domain, it will not resolve, and therefore not allow you to configure it. CNAMEs on the root domain are unsupported under RFC, however CNAME flattening resolves this limitation.
Portals should allow you to either:
a) add a domain without CNAME validation (perhaps an override option) OR
b) detect CNAME flattening and validate domains accordingly
The first option is probably more practical, as CNAME flattening is not easily detected from the DNS records.
Without this, it is technically impossible to serve a PowerApps portal at the domain root - as many clients require - meaning we end up having to put them on www with a redirect from the root domain, which is not ideal.
Many clients are now looking to use PowerApps portals as their primary websites - due in part to the great feature set and seamless integration with PowerApps/Dynamics 365. Now they want their domain root to resolve to them.