We need more information to understand what the error is. Check the system log, browser console to find details.
I tested the plugin using my sandbox account for nopCom 4.2. And although It is working ... BUT on the page, http://localhost:15536/Admin/PayPalSmartPaymentButtons/Configure after SAVE ... I got the message ... Webhook was not created, so some functions may not work correctly (see details in the log). Could you clarify where and how should I fix it? ...I didn't find any information about Webhook.