Are you using the portfolio post-type from the Jetpack plugin?
If you are then the post-type is named jetpack-portfolio, not portfolio.
Otherwise you need to navigate to your WordPress admin page and then on the left hand menu click the Portfolio item to get a list of all your portfolio posts. Once you’ve found the list of all your portfolio items you should see in the address bar an address similar to:
https://example.com/wp-admin/edit.php?post_type=portfolio-posttype
The post type for use in the short-code should be after post_type=, up to either the end of the address or any & characters (which are used to separate key and value pairs from each other, where the key is before the = and the value is after)
