The Schedule Call (Cycle_Plan_Schedule_Calls_vod) button is used to plan a Call from the Cycle Plan Target page. Why does an error appear: Invalid left-hand side in assignment?
This behavior is working as designed. The Schedule Calls button is designed to be used from the Cycle Plan page itself on the targets section. It is not designed to work from the Cycle Plan Target Detail page.
CRM Help Documentation: Default Functionality for Classic Cycle Plans