I would suggest using StackBlitz or CodeSandbox. It should be fairly obvious what template to pick depending on what you are using (e.g. plain JS or React).
are you saying you want to know how to use that “test cases via fcc test cases checker plugin?” then you can do so by using that “fcc test checker plugin” check this out Build a tribute page - #2 by bappyasif
That is about as simple as it can get and StackBlitz is a much better dev experience than Codepen. At least for framework code. And yes, you can use it for the certificate projects.
On Codepen you have to set the JS to use Babel under Settings > JS > JavaScript Preprocessor and you obviously need React added as well.
On your Codepen if you open the console you can see there is an issue with the imports. Also, ReactDOM is not from the same package as React.
Replace it with this instead.
import React, { useEffect, useState } from "https://esm.sh/react@17.0.2";
import ReactDOM from "https://esm.sh/react-dom@17.0.2";
Also, remove the React script you have under the Settings (don’t mix the settings scripts with imports)
Anyway, I would still very much suggest you keep using StackBlitz. Even if you are done with this and as such the dev experience doesn’t matter much now.