modifying OOB Workflow in Process Designer

Hi All,

I am new to PD and was exploring the same, I am trying to add a single validation for  a field in Incident Management module, however it is not allowing me to add a rule in the ruleset. I then copied the Incident worklfow and added the rule, however it is not working.

How can i achieve the same. Also can i relpace the OOB workflow with the copied workflow? If yes, what changes have to be made?

Thanks,

Krunal Parmar