How to fix WooCommerce thankyou page “not found” 404 error

For some WooCommerce users, the order confirmation page, or thankyou page, gives a 404 page not found after updating WooCommerce or their WordPress theme.

Here is the most universal solution that will work for most sites:
Add this code to your theme’s functions.php:

function flush_rules(){
flush_rewrite_rules();
}
add_action('init','flush_rules');

(Go to menu Appearance > Editor and find the file function.php in the right column.)

If this does not help, try to inactivate plugins one by one. It can also help to inactivate and reactivate WooCommerce.

Good luck!