I want to add opening balcances in customer's account but i can't due to column of opening balce is disabled

I've got different options for you on how to handle your customer's opening balance, @rameshmalik0203-.

If you haven't entered any amount on your customer's account, you still be able to add them by modifying it. To do that, just edit the account from your chart of accounts and add the balance. Please see the screenshot attached for your reference: 

However, if you've added an amount and you wish to edit it, you'll need to create a journal entry to record it. But before doing so, I recommend consulting your accountant to help you in verifying the accounts to be debited and credited in this process.

Once done, you may follow the steps found in this link in generating a JE: What to do if you didn't enter an opening balance in QuickBooks Online.

On the other hand, you can also create a credit note for the customer's balance, so any sales transaction made will be deducted from this credit.

To do that:

1. Select + New.
2. Choose Credit note. 
3. Enter the customer’s name, then fill out the required fields:
   • Date
   • Products/service items (you can create an item specific for this customer)
   • Quantity
   • Amount
4. Hit on Save and close. 

After that, the credit is now ready to be applied to your customer's sales transaction. For additional details about this procure, feel free to check out this article: Learn when and how to give a credit note or delayed credit to customers in QuickBooks Online.

I'll be right here to provide continued support if you have any other questions. It's my priority to help. Have a nice day!