prompt is something REPLit adds to its Node.js runtime environment. It's not part of Node.js itself.

I remember this not working for one of my nodejs repls.
I solved it by requiring readline-sync:

var readline = require('readline-sync');

You can get user input like this:

var name = readline.question("What is your name?");

If this doesn't work you might need to install it from the package manager first:

prompt is to take input
and in node.js to make variables you should write let or var before writing the name of the variable