When the voucher numbering method in your voucher is not set to automatic, this error occurs. Read on for the solution.
The entries of banking are posted into payment, receipt, and contra vouchers. If the voucher numbering method of these vouchers is set to "Manual" or "Automatic (Manual Override)" or any other method other than "Automatic," this error will occur: "Voucher Number cannot be left blank."
To check the reason for the entry failure, kindly refer here
Above is the error screen-shot from the Tally.imp file
For Tally Version 3 and above check All Exception for Error reason