error installing smush pro

Hi

I've installed smush pro but when I try to activate it I get this message
Fatal error: Cannot redeclare i18n() (previously declared in /var/sites/t/thespace.online/public_html/wp-content/plugins/wp-smushit/wp-smush.php:278) in /var/sites/t/thespace.online/public_html/wp-content/plugins/wp-smush-pro/wp-smush.php on line 276

When I tried to activate it via your dashbaord I got a server error 500 message.

I'm not a dev!! So keep it simple please