Hi,
Thank you for sharing the error and letting us know that you are using the latest version of the plugin.
I have tried to replicate the issue on our staging site with the latest version 2.5.4. But I am unable to replicate the issue as the checkout page is working fine nor any errors are recorded in the debug.log file.
Can you please let me know whether you have tried by switching the currently active theme to the default WordPress theme or the StoreFront theme?
If no, then can you please try and let us know whether the mentioned issue is fixed.
If the issue still persists then try by deactivating all the plugins except WooCommerce and the Payment Gateway Based Fees and Discounts for WooCommerce plugin and let me know the result.
Yet the issue persists then please send us the admin details of your site at support at tychesoftwares dot freshdesk dot com. We shall check the issue and get back to you with an update.