You've added prospects to a sequence but they're now in a Failed sequence state. The failed error message is: Malformed href - sender.calendar_url
Failed prospects in a sequence
Check the sequencer's calendar settings page and make sure the calendar is connected and authenticated.
The system is trying to calculate and populate the variable on that sequence step template that references the user's calendar URL. However, it cannot do this since the field is located on the user's calendar settings page, and if their calendar is not connected, the field won't be accessible to the system.