Registering devices with Intune for management and policy enforcement
Quick gut check before you go deeper on enrollment: that "Incomplete user enrollment" status is probably not your problem. That report just tracks people bailing out of the Company Portal wizard, and the app protection grant in CA doesn't actually care about device enrollment state at all, it's evaluated at the app level.
On Android, the thing that actually has to happen is Company Portal registering as the broker. A couple things I'd check first:
Did the user sign into Company Portal itself, not just Teams or whatever app they're trying to open? That sign-in is what completes the broker registration. Installing the target app alone won't trigger it.
Is Authenticator also on that device? If it landed before Company Portal, MSAL just talks to whichever broker got there first, and you can end up with a device that looks fine but never actually registers.
Worth confirming the app itself supports Intune app protection too. If it's not built on the Intune App SDK, no amount of broker fiddling gets you there.
Fastest way to tell what's actually going on: Apps > Monitor > App protection status > Android user status in the Intune admin center. If there's no check-in at all for this user, you're looking at a broker registration issue, not a policy assignment one.
Doc for reference if you want to go deeper: https://learn.microsoft.com/en-us/troubleshoot/mem/intune/app-protection-policies/troubleshoot-mam