Server error status: Internal Server Error [500] when trying to install WP Smush Pro

I just got my account because I want to use WP Smush Pro on the websites I operate. I downloaded the WP Smush Pro plugin, but when I try to activate it, I get the following error message: Server error status: Internal Server Error [500]

What should I do to get it activated?

Nevermind... I deactivated the free version and that let me install the Pro version.