Hi,
The Express Checkout payment plugin doesn't work as a normal payment plugin as the goal of PayPal's Express Checkout is to skip the website's checkout to use PayPal's.
So instead of displaying as a payment method in the payment method selection area, it instead has display settings to display an Express Checkout button on the cart view of the checkout and on the HikaShop cart module you can configure via your Joomla modules manager.
If you don't plan on having other payment methods, you can remove the line
$this->checkoutHelper->displayMessages('payment');
from the show_block_payment view file via the menu Display>Views to remove the error message.
If you don't see the Express Checkout button on the cart view of the checkout, then it indicates there is likely a problem with the settings of the payment method.