I have a no series of sales invoice, some no are getting skipped while creating the invoice
i am using Navision 2013 R2
How are your number series setup? I know in some companies/countries they may use the same number series for invoices as for credit memos. So start by checking your setup.
Both posted and un posted are having same no series
You mean ALL 4 fields i Sales and Receiveables Setup are having the same code? If that is true - then it must be as Erik Suggests because a creditmemo has been created/or posted
If that is not the case, then you have to do a printscreen so we can give you the answer..
Credit memo having the same code only.