Hello Seth,
the error message is a bit misleading.
But you will need to post an ID (topObjID) of the object the note should be attached to as well as the object type (topNoteObjCode).
"body": {
"noteText": "Test 001",
"topObjID": "<object_id>",
"topNoteObjCode": "TASK",
"json": "{}"
}
Regards
Lars