getting error in Script task element in Business process.

I am trying to design the Business process in Creatio where i am trying to read  response returned by 3rd party web service which  returns the response in XML.  

For processing the XML response, i have added the new  class (source code) in my package with name CustomerHelper which i added in my package.

now when I add Script task element and add below code into it, it shows error as below.

Code from script task :- 
    
            string xmlResponse = Get("ResponseAPI");

            var names = Terrasoft.Configuration.CustomerHelper.ParseCustomers(xmlResponse);

            Terrasoft.Configuration.CustomerHelper.CreateAccounts(UserConnection, names);

            return true;

Error I received as given below.

The type or namespace name 'CustomerHelper' does not exist in the namespace 'Terrasoft.Configuration' (are you missing an assembly reference?)    CS0234    37
The type or namespace name 'CustomerHelper' does not exist in the namespace 'Terrasoft.Configuration' (are you missing an assembly reference?)    CS0234    35

This same process worked for me using Creatio cloud but I am getting above error when I do it in Creatio On premise.

Does anyone have any suggestion what i am doing wrong while doing it in Creatio On-premise