Please make sure you have put the correct integration ID & key corresponding to the currency you set in your woocommerce settings. If you set USD make sure your integration id and keys are usd. You should do likewise on ZWL
I managed to work out it’s probably a blocks error and Paynow integration not yet being supported! I worked around it by editing the template page or creating a new checkout page and using the shortcode block: [woocommerce_checkout]