Not sure what is going on, Replit is installing the wrong version of the dependencies.
Open the Shell tab next to the console tab below the preview window.
From the Shell tab run npm rm mongoose && rm mongo
From the Shell tab run npm i mongoose@5.11.15 -E
You have to remove this document.getElementById("content").innerHTML=data; from the route handler. Node.js is not the browser and you do not have access to the browser web APIs.
Run the Replit and try the submit again, it should work now.
Edit: I just tried replicating it again using a fork of your project and it failed.
But if I start a new boilerplate and just copy and paste your code in (with my own DB) it passes the tests. So I’m not sure what is going on.