It's probably just a generic error message for failing that Google Play Protect thingamajigger that attests provenance of the vendor OS from boot. Will be interesting to see whether the Motorola phones have this endorsement when they ship. Most devices would probably fail because they are rooted rather than because they are GrapheneOS. I wouldn't put it past a scammer talking grandma into rooting their phone.
It isn't due to the Play Integrity API. That shows a notification on GrapheneOS with a toggle for blocking it to work around it for services not enforcing providing a result. If that was the issue, the original poster would have known from the notification. The issue ended up being PayPal shipping incorrect anti-tampering code incompatible with secure spawning. The original poster figured that out and got it working by disabling the per-app secure spawning toggle.
It will be possible to install the official GrapheneOS on the devices. Shipping devices with the official GrapheneOS is planned but may not be there at launch.
