Anyone using this with Numinix FEC? I have a client with FEC 2.4.1 and Zen Cart 1.5.6c

I installed this new plugin and now get this: https://screenrec.com/share/jA5gWp0aSb
One error in the console but about a missing flexslider function

I tried putting jscript_square_webPay.php in /public_html/includes/modules/pages/checkout When I do, this button disappears.

When I turn off FEC, it seems to work fine.