Insert Shortcode button is missing

Since version 5.0.7 minimum user capability for the “Insert Shortcode” button was raised from edit_posts to manage_options, which means only users with this capability can see and use the button. Usually, this capability is only available for Super Admin and Admin account types.

Important Note
This was made to improve plugin security. You can do almost everything with shortcodes. SU allows you to load custom php scripts, display administrator’s email address and much more. That’s why regular users are not allowed to use it.

If you want to allow Editors or other non-admin users to use the button, paste the following snippet to the end of the functions.php file of your active theme:

/**
 * Replace the 'editor' with a value you need. For example:
 * 'edit_others_posts'. User roles and custom capabilities
 * are allowed.
 */
$su_generator_capability = 'editor';

add_option( 'su_option_generator_access', $su_generator_capability, '', false  );

Once again – I do not recommend doing this!

Further reading: WP Codex: Roles_and_Capabilities