sends a message. But, the assignment is that the message needs to be uppercase based upon the process.env.MESSAGE_STYLE variable. So, modify the message value before sending the message.
Your code can be as follows and this needs to be within the GET request so that when the request is made in the browser with this url http://localhost:3000/json the response is displayed depending upon the MESSAGE_STYLE.
let myMessage = "Hello json"
if uppercase
myMessage = myMessage converted to uppercase
res.json({ message: myMessage })