Hello,
I am Daria, technical support manager for PrestaShop module Paypal.
One Page Checkout modules that display the purchase tunnel in a single page may cause incompatibilities with the Paypal module as well as PS checkout module.
In most cases, these modules redefine a part of the checkout tunnel, making it inoperative to update payment amounts following the carrier selection. However, this operation is essential for Paypal to receive the correct order amount. It also happens that these modules cause Javascript errors that hide or make payment impossible.
This problem is not a malfunction of the module, but a customization of the purchase tunnel.
I suggest you to contact Prestashop support team for a possible override or the developer of the OPC module for compatibility modification.
I also invite you to try our Paypal module, for which we can provide an OPC compatibility override
https://addons.prestashop.com/en/payment-card-wallet/1748-paypal-official.html
Regards,
Daria - Support 202 e-commerce