Hello!
This message might indicate, that in a project you’re trying to create Test Sets in, there is a Custom Field called “Project”, and it is a Mandatory field on a Test Set level.
In this case, you have to fill this field with a value each time you send a POST request of Test Set creation.
In order to check and retrieve this field’s system ID, you can use the following request: Full API Documentation (Version 2) - PractiTest