Pluggin Can Not Be Activated

After installation and attempt to activate, this error appears:

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, class InboxWebmail_Helper_Upgrade does not have a method "upgrade" in /srv/www/domain.com/www/wp-includes/class-wp-hook.php:303 Stack trace: #0 /srv/www/domain.com/www/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array) #1 /srv/www/domain.com/www/wp-includes/plugin.php(470): WP_Hook->do_action(Array) #2 /srv/www/domain.com/www/wp-admin/plugins.php(193): do_action('activate_wp-inb...') #3 {main} thrown in /srv/www/domain.com/www/wp-includes/class-wp-hook.php on line 303