This issue can be caused by multiple settings including having the Sandbox (test) Turned on in your plugin.
Here is a 3rd Party Website (not created by PayPal) that has a listing of the most common PrestaShop issues with the PayPal integration.
https://www.presto-changeo.com/content/10-fixing-paypal-problems
You may need to turn on Instant Payment Notifications in your PayPal Account.
Here is the information that Prestashop has on the Instant Payment Notification URL:
https://www.prestashop.com/forums/topic/102759-what-is-the-prestashop-paypal-instant-payment-notification-ipn-url-please/
Here are the steps to turning on Instant Payment Notifications in your PayPal Account (Notifies Prestashop when an order is complete)
Infocenter
With Instant Payment Notification, PayPal sends payment notification messages with encrypted code to the specified URL for each payment you receive.
Log in to your PayPal account.
Click the Business Profile icon beside "Log Out" at the top of the page.
Select Profile and Settings.
Click My selling tools.
Click Update beside "Instant payment notifications" in the "Getting paid and managing risk" section.
Click Choose IPN Settings.
Enter the notification URL and select Receive IPN messages.
Click Save.
Here are the general integration instructions for PayPal and Prestashop:
https://www.paypal.com/us/selfhelp/article/how-do-i-integrate-paypal-payments-standard-with-prestashop-ts1822
Thank you,
Jennifer
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