Basic activate question: Membership plugin won't activate

I have an existing self-hosted WP site, and uploaded the full Membership plugin to the plugins folder.

When I go to activate the plugin, it tells me that there is a “fatal error.” I remember seeing somewhere that with an existing WP site you have to do something BEFORE you activate plugin, but I can’t find this info anywhere now. It isn’t in the manual and I can’t find it online.