The system should not allow the release of Project/WBS element if the settlement rule is not assigned

Dear PS Gurus,

my requirement is that The system should not allow the release of Project/WBS element if the settlement rule is not assigned & system should give the error message. Please let me know the procedures....

steps-

project creation- cj20n

settlement rule not maintained

releasing of project- now system is allowing to release the project. Here system should not allow to release the project and give an error message....

please help

prashant