Overview:
A user is not able to send a review task to a custom group in Vault.
Root Cause:
The lifecycle user role is not configured for All Users and Groups.
Solution:
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).
Related Documentation:
Vault Documentation: Vault Help