The “token” you have created called decode is a new API Key that is not the same as an app API key that you need to decode a token passed to the app.
To create an api key for the for an you want to follow the setting up your first app instructions and use the api key generated here in your env variables.
Keep in mind the token you were using previously wont work with this api key. You will have to get the token by viewing the new app that you added.