pixelcasualuser I'm assuming that it detected some kind of virtualization/sandbox.
That seems unlikely, since GrapheneOS doesn't run Venmo inside of virtualization and because every version of Android runs Venmo inside of the Android app sandbox.
Perhaps the Venmo app on your device detected something, but perhaps Venmo the company didn't like something about the account information provided.
A web search for site:discuss.grapheneos.org Venmo might turn up something useful.