It is important to be consistent in defining process owners (usually an organization) and other roles in your diagrams. Using Pools facilitates this.
If you want to represent responsibilities in a process, the correct usage of Pools and Lanes is important. To comply with this rule, all modeled elements must be contained within a Pool.
If this error is displayed, check if all modeled elements are contained within a Pool. This is necessary so that every element has a responsibility value assigned.