I have two change activities, CA1 and CA2 created based on the CustomerRelease. I have updated the discount rule in CA1 to give a discount to a product, and this CA1 is not closed. When I go to the CA2 with the discount rule to this product, this rule is locked. Why this happen?

To prevent multiple user making changes to the same discount rule, an activity lock is created in the change activity level at the CA1. By doing so, only one person can make changes to the master copy of the discount rule. If another person would need to make changes to the discount rule, the existing CA1 needs to be completed and another CA3 needs to be created in order for it to be created based on the latest CustomerRelease. Or CA1 needs to be canceled in order for CA2 to make changes to the discount rule.