Update process variable at runtime

There's likely two problems here. First, you can only use a!save() in an interface. If you want to save into a process variable, you should use a script task and add a Data > Output. Using the output you can create any expression and save the results into a process variable. The second issue is that whatever node you're using doesn't have the correct kind of form. If you truly are just updating a process variable directly, you shouldn't have a form at all.

If you still have questions, can you provide more information about your configuration? Where did you configure that expression? What does your process do? Can you share screenshots of what you configured?

Is there any way I can identify how may times this tag (Schedule_1) was invoked in one complete process flow. Or which instance is running currently (at runtime) ?  

Like I found tp!instanceindex under Task Properties which gives me how may tasks have been / has to be executer.  

I agree with Mike's point above, but in general I'm also not sure you're thinking about these tags correctly. "Schedule_1" is just an input - it determines what value to insert into your Word doc, but it isn't truly a variable. So there's no way to get information about when it was invoked or used - if you complete this node whatever value you provide is always inserted.

Are you trying to create multiple documents at once? Why are you trying to track information like this?

I agree with Mike's point above, but in general I'm also not sure you're thinking about these tags correctly. "Schedule_1" is just an input - it determines what value to insert into your Word doc, but it isn't truly a variable. So there's no way to get information about when it was invoked or used - if you complete this node whatever value you provide is always inserted.

Are you trying to create multiple documents at once? Why are you trying to track information like this?