It simply means that data is not sent as per documentation. I was facing this issue as in my case "contents" parameter was passing empty string, instead of documentation says that "contents" parameter should contain atleast 2 parameters, that is, quantity and id.

Go through documentation and you can compare your value format with documentation.

Here is link for documentation :- https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/custom-data#content-ids