Membership 2 Authorize.net - Class not loaded error

Hi - I'm helping with payment gateways on a WP site that has Membership 2 installed. We have set up Authorize.net but the transaction is failing when purchasing a membership with the following errors:
Sorry, your signup request has failed. Try again.
Class not loaded: ReallySimpleCaptcha
Class not loaded: WooCommerce

I enabled the debug log and when I retry the transaction, I see the following (7 times):
[08-Mar-2017 12:43:16 UTC] PHP Notice: Undefined variable: subscription in /var/sites/i/imaginalhealth.com/public_html/wp-content/plugins/membership/app/model/class-ms-model-membership.php on line 2348

Thanks for your help,
Bill