undefined is not an object(evaluating 'RNFS.writeFile')

I am trying to create a mobile app with react native which checks does the user have an id or no in the start of the app, and if the user doesn't have an id then it gives the user an id and updates a json file. I am trying to do it like
```js
const RNFS = require("react-native-fs");
console.log(RNFS);
const settingsFile = require("./app.json");
if (settingsFile.userid == null) {
const newUserid = Date.now().toString(36) + Math.random().toString(36).substr(2);
settingsFile.userid = newUserid;
RNFS.writeFile("./app.json", settingsFile, "utf-8")
.then((success) => {
console.log('FILE WRITTEN!');
})
.catch((err) => {
console.log(err.message);
});
}
```
But I am getting the errror `undefined is not an object(evaluating 'RNFS.writeFile')`