1. Log into Wordpress admin
2. go to Pages
3. Edit Checkout page
4. In the editor page, scroll down to "Payment options" and click on "Configure Payment Options"
5. On right column, there will be a warning saying "Incompatible with block-based checkout" and a button "Switch to classic checkout"
6. Click the "Switch to classic checkout" button and click "switch" in the popup dialog
7. Click the "Save" button on the top right for save changes