Get posts with WP-REST without authentication
You actually don't need to have authorization to merely GET posts or categories, so long as they aren't password protected.
If someone has implemented a modifier to rest_authentication_errors, that might be your problem: https://developer.wordpress.org/rest-api/using-the-rest-api/frequently-asked-questions/#require-authentication-for-all-requests
To see the available endpoints you have, visit yourwordpressurl.com/wp-json