Rpc error: code = Unknown desc = Input for predicate author of type uid is scalar

as here

{
   "_Board": " ", # type as Board
   "Title": "My Title",
   "Description": "My Description"
   "Author": {
     "uid": "0x4" # Existing user
   }
}

*updated

Note A common mistake is to attempt to use {“uid”:“0x123”,“link”:“0x456”}. This will result in an error. Dgraph interprets this JSON object as setting the link predicate to the string"0x456", which is usually not intended.