WP Multisite Activation Breaks Subsites

Just a quick note, that on WordPress Mutlisite, this plugin prevents subsites from loading if activated, both via network activate or activating on a single subsite.

Network Activate prevents all subsites from loading. Single subsite activation prevents the site from loading for that single subsite.

My network is setup as subfolders and not subdomain.

There isn’t an issue with the main domain, it just breaks the subsites.

I turned on Debug, and this is the error code that returned:

Fatal error: Call to undefined function is_checkout() in /home/skookumm/public_html/wp-content/plugins/express-checkout/includes/class-express-checkout.php on line 244