how to fix this error TypeError [ERR_INVALID_CALLBACK]: Callback must be a function

Fs.writeFile() according to the documentation here takes ( file, data[, options]and callback ) params so your code will be like this :

 var fs = require('fs');
 fs.readFile('readMe.txt', 'utf8', function (err, data) {
  fs.writeFile('writeMe.txt', data, function(err, result) {
     if(err) console.log('error', err);
   });
 });

Since node 10, it is mandatory to pass a callback on fs.writefile()

Node.js documented the purpose for the change: https://github.com/nodejs/node/blob/master/doc/api/deprecations.md#dep0013-fs-asynchronous-function-without-callback

You could add an empty callback like this fs.writeFile('writeMe.txt', data, () => {})

fs.writeFile(...) requires a third (or fourth) parameter which is a callback function to be invoked when the operation completes. You should either provide a callback function or use fs.writeFileSync(...)

See node fs docs for more info.