The documentation portal is undergoing maintenance. During this period, you can't create any new user credentials. If you do, you'll need to recreate the user credential after the maintenance period ends.
Often, you can use an automatically generated Serializer directly. If required, you can edit the auto-generated Serializer to correct any limitations or problems that you find in it.
The following paragraphs list some typical circumstances under which you need to edit the Serializer, and the suggested editing steps.