Incomplete invoice routing participant

Summary:

We are looking to better utilize the Incomplete Invoice Routing hold for our business. We are looking to have the system auto assign the invoices to a group users based volume. Example would be if we import 10 invoices and there are 5 users in the group the system would assign 2 invoices to each user. An option we have seen under the Advanced options of the default assignee is Enable auto assignment: User and Assignment pattern: Least Busy but it is greyed out. We can't figure out how to enable this function or edit the