Assign a role to the group using cloud resource manager api in gcp. I'm new to gcp, my understanding is of beginner level

Question

As the title states I'm assigning a role to a group using cloud resource mananger api using python client library. Here's the code :

from googleapiclient.discovery import build
from googleapiclient import errors
 
service = build('cloudresourcemanager', 'v1', credentials= delegated_credentials)

def assign_custom_role_to_group(project_id, role_name, group_email):
    try:
        # Define the request body
        policy = service.projects().getIamPolicy(resource=project_id).execute()
        #print("GET IAM", policy['version'])
        bindings = policy.get('bindings', [])


        # Add the custom role to the bindings
        bindings.append({
            "role": role_name,
            "members": ["group:group_email"],
            "etag" : "BwXy6qC04wk=",
            "version" : 1
        })
        policy['bindings'] = bindings

        # Execute the request
        service.projects().setIamPolicy(resource=project_id, body=policy).execute()

        print(f'Successfully assigned custom role {role_name} to group {group_email}')
    except errors.HttpError as error:
        print(f'An error occurred: {error}')

For this code I'm getting the following error. enter image description here. Thank you.