Unable to do Lead Run assignment REST API from Groovy script

Summary:

We have a use case to run the lead assignment ( To assign the territories) on lead create event.

We are trying to call the Lead Assignment REST API from a Groovy script on the Before Insert event. However, we are encountering errors while invoking the API. We have attached the Groovy scripts and the corresponding error messages for your reference.

P.S : Same REST API works from VBS but we are not able to achieve this on lead create event since the LeadId will not be available until the lead is created hence we are trying to achieve this using Groovy scripting.