Groovy error "A method called by the script failed on line..."

Summary:

I am trying to update the substitution variable using a groovy script from the Business rule, which is returning an error.

Content (required):

I have a groovy script that uses rest api to call a business rule then is suppose to update the substitution variable and checks the run status until the business rule completes. It appears to fails

Error message: A method called by the script failed on line XX (which refers to the line bolded below)

//------------------ Application Details---------------------//

String connectionName = "CurrentApp"

String appName = "OPEX"

//--------------------To get current month, last month, Current Year and last year------------------//

def curr_date = new Date()   //Load Current Data Object