I am new in Magento. I want to add more fields in onepage checkout billing and shipping address.
I have followed this Link
It would be better if you provide some more information like Magento version & billing.phtml file.
I would recommend to try some readyy-made extensions like Magento One Page Checkout Custom Fields extension by FME which allows you to add custom fields to any section of the checkout page. Demo: https://www.fmeextensions.com/magent...es-fields.html