How we can add Opening Balance of Customer and Vendors.
This is explained very clearly in the Set Starting Balances guide
The entire scheme for starting balances changed two days ago. The main Guide was updated yesterday: https://www.manager.io/guides/15718. The more detailed Guides on starting balances for customers and suppliers referenced in that Guide still refer to the old version. The Guide web site updates once per day. So the revised versions of the customer/supplier Guides should be posted in a few hours.