Invalid Client is usually due to a bad request, not a bad client id.
In this case, make sure you’re following the Client Credentials OAuth flow as documented Getting Tokens: OAuth | Twitch Developers, which should be done from your server and not from a browser.
Additionally, as subscriptions is private data you’ll also need to use either the Implicit or Auth Code OAuth flows with the user you wish to get notifications for so that they grant your app that permission, otherwise when you try to use the App token to subscribe to EventSub for notifications it will deny you the ability to listen for subscriptions.