JavaScripts error in page

Hi,

great plugin, but I’m not even allowed to start one level.

https://www.suonareaorecchio.com/account-iscrizione/pagamento-iscrizione/?level=1

It’s italian, but I guess you can figure out the situation. The red string says “JavaScritp error. Contact the webmaster”

What I did is:

1) Installed the plugin

2) Automatically created the pages

3) Set up a paypal express payment (sandbox)

When I try to make a sandbox payment, the red string appears.

Can you help me with that? Am I supposed to pay 99$ just for this?

Thank you

https://www.ads-software.com/plugins/paid-memberships-pro/