Unfortunately, the category is one of the fields that cannot be created or updated thru the kintone REST API; though, the values can be retrieved from it. *This is mentioned in one of the limitations of the kintone REST API.
-kintone REST API Overview
If you would like to bulk update the existing records, maybe consider using the "Importing Records from a File" feature. You can use this to bulk update, including the category field, the existing records, or to create multiple records.
-kintone Help Center - Importing Records from a File
I hope this helps.