The ACH Return Code R 01 signifies Insufficient fund.
To clear any ach transaction the payers account must have the requested amount of funds. If the available disposable funds are not equal to the request funds then merchants will see ACH Return Code R 01
So what is the solution of ACH Return Code R 01
Since the funds are insufficient the solution is simply to see if the payer can add the remaining funds to clear the transaction. After having required funds in the account the transaction should go through.