WooCommerce POS will give a timeout error if an order takes longer than 1 minute to complete.
This could be caused by:
Your internet connection, e.g.: slow connection, no connection
Your server, e.g.: high load, server down
Your gateway, e.g.: connection to gateway server, fatal PHP error
Your plugins, e.g.: connection to a third party service such as Xero
Please note that WooCommerce POS is just a frontend for the WC REST API. Once the order is sent to the server the POS plugin has little impact on the time it takes to complete.
The following plugins are known to cause problems with Time Out error:
For more information and some tips for troubleshooting, please see the POS Performance doc.