[Vuejs]-How to access property of stringified JSON?

You forgot to return the response in your last .then callback. So your const response is actually void.

const response = await fetch(
    "http://localhost:5000/create-checkout-session",
    // [...]
  )
    .then((response) => response.json())
    .then((response) => {
      console.log("stringied response", JSON.stringify(response))
      //  Return `response` here, or the Promise will return the returned value of `console.log` which is `void`!
      return response
    });