Checkout failures after a theme update usually happen due to outdated template overrides or broken JavaScript dependencies.
WooCommerce allows themes to override core templates, and these can become incompatible after updates.

Check WooCommerce → Status → Templates to see if overrides are marked as outdated. Updating or removing those files often restores checkout functionality.

JavaScript errors can also block checkout submissions. Open the browser console and look for errors related to checkout.js or jQuery. Conflicts commonly arise when themes bundle their own outdated scripts.

Many developers forget to test checkout flows after theme updates because the frontend “looks fine.”
The takeaway is to treat checkout as a critical path and test it after every theme or WooCommerce update.