A user is not able to send a review task to a custom group in Vault.
The lifecycle user role is not configured for All Users and Groups.
If the affected lifecycle role is configured to allow for All Internal Users to be added by default when starting a workflow, then groups cannot be added to this lifecycle role when starting a workflow.
The solution is to edit the affected lifecycle user role configuration so that it can be assigned to users and groups (All Users & Groups is a good alternative for All Internal Users).
Vault Documentation: Vault Help