The reason why this is happening is, most likely, that the Jetpack plugin is not installed and/or activated.
To check whether the Jetpack plugin is installed and activated, go to Plugins → Installed Plugins.
If it is installed and activated, go to the Jetpack option in the admin panel (you can find it under yourdomain.com/wp-admin/admin.php?page=jetpack#/) and click the Set up Jetpack button. After the Jetpack plugin is all set up, go to Jetpack → Settings → Writing and turn on Portfolio post type.
If the plugin is not installed or activated, be sure to install it and go through all the previously mentioned steps. If you need help with Jetpack installation and activation, feel free to check out this post.