WooCommerce has replaced the old PayPal Standard Payment Gateway since version 5.5.
The new PayPal plugin however is, at time of writing, not very popular and contains many bugs. One of them is, that negative Fees (used in Coupon Plugins or our own Reward Points plugin) are causing issues with PayPals API.
To use the old Standard PayPal gateway simply add this line of code into your child theme.
add_filter( 'woocommerce_should_load_paypal_standard', '__return_true' );
After adding this, go to WooCommerce > Payment > Enable PayPal Standard.
Learn more why the old Gateway will be hidden here.