Hello ​@Nagarjuna 

Clear revenue/cost balances functionality sums up revenue and cost account and create a voucher using account from GP8 posting. If you already created a voucher to transfer P&L balance to retained earning or P&L balance 0, this means your P&L sum is 0 and the function will not create any voucher. If you still want to zero the P&L balances then use one of these options

1. Export revenue and cost balances from GL and multiply balances by -1. Create an external voucher and import the balances
2. Create first a voucher between retained earning and any P&L account to change equation in the P&L. Then you can run the clear revenue/cost balance functionality
3. run periodical cost allocation to transfer balances from cost and revenue accounts to retained earning

hope this helps