I have been reading this page, which is a tutorial for the .itch.toml file.
I understand most of it however the Scoping part is too vague.

It says something about the "scope" parameter, however it never said what a parameter is.

And how am I supposed to debug my game with the manifest file?

Hey, just answered to your github issue as well :)

The "scope" parameter is just the "name", "path", or "args" parameter for actions. See this file for example: https://github.com/fasterthanlime/sample-evil-app/...

To test your manifest file, you'll have to push a build of your game to itch.io - ideally, using https://itch.io/docs/butler/ - so that you can push any changes quickly instead of having to reupload the whole thing.

Then, simply install and launch your game from the itch app. You can open the game's "event log" (which contains information about problems with the manifest, among other things) by right-clicking the game's tab, opening "View log":