Like the error message suggests, you probably just need to specify a region in your .env file or filesystems config. You may also need to specify the url.
Your region should be visible in the AWS/S3 console and the available values are listed here: https://docs.aws.amazon.com/general/latest/gr/rande.html#cf_region
Relevant StackOverflow post: https://stackoverflow.com/questions/39392114/laravel-and-aws-cloudfront