It's possible to set up rules in Palette to ensure that each invoice is signed by more than one user/person. The users that signs the invoice may need to have different functions in the company, such as one approver and one manager, or someone from Accounts Payable and one manager.
If you receive this message, it means the invoice needs to be sent to at least one more role. The group that is mentioned in the message is the group that the invoice has already been signed by, in this example Manager. Another type of role needs to be added to the flow.