1) Check at invoice entries whether this running number already used there
2)If cannot found, check at QMT
3) GO to File -> Utilities -> QnE Management Tools -> Intelligence SQL Queries -> Check field : Invoice -> Execute SQL -> find that running no at these field .
4) If cannot found repeat steps 3 for field InvoiceDetails, SalesInvoice, and SalesInvoiceDetails
4) If can found the running no for example at Invoice Details field, delete it. Below is the script to delete :
delete from INVOICEDETAIL
where INVOICECODE = 'AA123'
5) Tick Overwrite Generated SQL -> paste above script -> click Execute SQL