PowerBIClient.Gateways.GetGateway returns Unauthorized error

Trying to configure datasource credentials at runtime and the call to PowerBIClient.Gateways.GetGateway returns Unauthorized error.  Below is the snippet of code used to do this:

Dim basicCredentials As BasicCredentials = New BasicCredentials(username, password)
Dim gateway As Gateway = powerBIClient.Gateways.GetGateway(datasource.GatewayId)

Dim credentialsEncryptor = New AsymmetricKeyEncryptor(gateway.PublicKey)
Dim credentialDetails = New CredentialDetails(basicCredentials, PrivacyLevel.Private, EncryptedConnection.Encrypted, credentialsEncryptor)
Dim udsRequest As UpdateDatasourceRequest = New UpdateDatasourceRequest(credentialDetails)
powerBIClient.Gateways.UpdateDatasource(datasource.GatewayId, datasource.DatasourceId, udsRequest)

The above code was found in the link:

https://docs.microsoft.com/en-us/power-bi/developer/automation/configure-credentials?tabs=sdk3

Any help would be appreciated.

Thanks.