Cannot update Google BigQuery data source credentials using the API

I'm trying to update credentials of a BigQuery data source using the sample code from Microsoft that's available in the link below. 

I'm using a service account with basic credentials to connect to BigQuery. The username and the password works when I enter from the Power BI website. However, I get an Invalid username and password error when I enter the same credentials to the webapp that the sample python code launches.
I'm using a service principal to authenticate with the Power BI REST API and it is an admin for both gateway and connection. Much appreciated if someone could shed some light on to what's causing the issue.

Sample Code: https://github.com/microsoft/PowerBI-Developer-Samples/tree/master/Python

Error:

{'error': {'code': 'DM_GWPipeline_Gateway_InvalidConnectionCredentials', 'pbi.error': {'code': 'DM_GWPipeline_Gateway_InvalidConnectionCredentials', 'parameters': {}, 'details': [
                {'code': 'DM_ErrorDetailNameCode_UnderlyingErrorCode', 'detail': {'type': 1, 'value': '-2147467259'
                    }
                },
                {'code': 'DM_ErrorDetailNameCode_UnderlyingErrorMessage', 'detail': {'type': 1, 'value': 'The credentials provided for the GoogleBigQuery source are invalid. (Source at GoogleBigQuery.)'
                    }
                },
                {'code': 'DM_ErrorDetailNameCode_UnderlyingHResult', 'detail': {'type': 1, 'value': '-2147467259'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.CredentialError.DataSourceKind', 'detail': {'type': 1, 'value': 'GoogleBigQuery'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.CredentialError.DataSourceOriginKind', 'detail': {'type': 1, 'value': 'GoogleBigQuery'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.CredentialError.DataSourceOriginPath', 'detail': {'type': 1, 'value': 'GoogleBigQuery'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.CredentialError.DataSourcePath', 'detail': {'type': 1, 'value': 'GoogleBigQuery'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.CredentialError.Reason', 'detail': {'type': 1, 'value': 'AccessUnauthorized'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.MashupSecurityException.DataSources', 'detail': {'type': 1, 'value': '[
                            {
                                "kind": "GoogleBigQuery",
                                "path": "GoogleBigQuery"
                            }
                        ]'
                    }
                },
                {'code': 'Microsoft.Data.Mashup.MashupSecurityException.Reason', 'detail': {'type': 1, 'value': 'AccessUnauthorized'
                    }
                }
            ], 'exceptionCulprit': 1
        }
    }
}