Now, to be able to help you, you must provide some kind of information that enables us to understand what happens. You can provide the code or the error shown when trying to require the package (helmet).
Nevertheless, in glitch, click on the package.json file and then, on the top left of the editor will be a button that says Add package, click it. Now, on the search for a package input, type helmet and then click on the result.
Now you should be able to require it:
// server.js
const helmet = require('helmet');
// Rest of the code goes here.
It’d be helpful to see the code, but the issue might be the URL you take from Glitch. Be sure to paste the URL describing the live app, not the project page.
Project page: https://glitch.com/~project-name
Live App: https://project-name.glitch.me
It seems the testing suite expects a link to the working application.