Cannot pass the POST request to Find query of mongolite and it returns null

Question

library(jsonlite)
library(mongolite)
library(lubridate)

#' @post /predict

  predict= function(x, y)
    {
            
    mongodatas <- mongo(collection='db12', url='mongodb://ganesh123:[email protected]:63137/ganesh123')

    naan <- mongodatas$find(query= '{ "data_time":  {"$gte": "x" , "$lt": "y"} }',
                          fields= '{"_id": 0, "data_time": 1, "id": 1, "sensor3": 1 }',
                          sort=  '{"data_time": 1}' ,
                          limit=5000
                          )

    return(naan)

  }

The above code is my R Api done using plumber and done using POST method and this works if the below postman input x and y is given as static in the find query

naan <- mongodatas$find(query= '{ "data_time":  {"$gte": **"x"** , "$lt": **"y"**} }',  "

    {
        
    "x": "2018-07-25 17:02:17.249591",
    "y": "2018-07-25 17:02:17.674801"
        
}

The above code is my input to R Api This is input from postman link http://localhost:3000/predict

 "id": "1",
 "data_time": "2018-07-25 17:02:17.249591",
 "sensor3": "4.12"

This is my database structure. The query cannot process the input data.