Issues using Update Constant smart service

Hi folks, 

I am trying to update a constant value, taking the value from an interface. For this I am doing the following steps: 

1. Using a dropdown field to input the value from the user and saving it in a local variable (cannot use a ri variable as the design is that way) 
2. Triggering a process model through the interface and passing the value of local variable to the pm
3. Using the update smart service to save the value in the constant 

But, in this process I am getting the following error:

Any help would be appreciated. TIA!