Membership Plugin Fatal Error


I am attempting to install the membership plugin on a WordPress website (not multisite) and I am getting the following error when I activate plugin:

Plugin could not be activated because it triggered a fatal error.

Fatal error: Cannot redeclare class paypalexpress in /home/incoaoc/public_html/wp-content/plugins/membership/membershipincludes/plugins/gateway.paypalexpress.php on line 622

The site is running WordPress in single site mode, and has the Shopp plugin installed on the website. Is there a known issue with the Shopp plugin that may be causing this paypay conflict?

Has anyone else seen this error when activating the plugin?

I have checked the WP error log and it's clean.