Check the Pymt Method dropdown (below the cheque itself & above the detail sections)
Are you actually sending an electronic (ABA) File from the software directly to the bank? This is what the "Bank (Online)" option means.
If you're just paying via logging on to your Internet Banking & making an EFT or BPAY from there, choose "Cash/Cheque" instead. This then makes the "No." field editable where you can either enter transaction-specific numbers as generated by your bank or just use generic combinations eg "EFT" "BPAY" etc