You’re using the wrong type of token.
The API endpoint you’re using gets the list of followed streams based on the user associated with the OAuth token. App Access Tokens don’t have a user associated with them as you’re not sending a user through the OAuth flow asking them to explicitly grant the required scopes.
Use the Authorization Code Flow https://dev.twitch.tv/docs/authentication/getting-tokens-oauth#oauth-authorization-code-flow and this will get you a User Access Token, which will let you access endpoints requiring permissions to be given by that user.