This issue occurred due to following reasons- 
  • Stationed Technician checkbox might be ticked by default that's why service schedule status was True
  • Service was already scheduled that's why Plan pop-up was not editable
Solution-
1. Untick Stationed Technician checkbox (due to which service(s) get scheduled automatically) in contract screen
2. Reschedule the service to any date which is already scheduled (which has schedule status as True) and after that refresh the customer service list
3. After refreshing the list, schedule status gets converted to False
4. Click on Plan button and a pop-up will open which is now editable 

5. Select required things into that pop-up and save the same by clicking on Ok button