We want to prevent a bill to be added in the system if no PO is attached. I prefer using a workflow. But all info as I am aware of is on line level. Don't know which criteria to set... I want to return a user error before user submit

Are you sure you want to do this? How do you guys normally bill for utilities like water or gas bill?

I am sure about the use case. We need it for various reason. We are aware of all implications. All fixed costs that we are normally billed for like water and gas will have a PO