Prepayment Invoice

Microsoft Dynamics NAV
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Prepayment No. Series must have a value in Sales Header: Document Type=Order, No.=RSO/14-15/00001. It cannot be zero or empty.
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OK
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Hello,

I am trying to post prepayment invoice , the above error is displaying...

I have setup number series for Posted Prepmt Inv Nos in Sales & Receivables Setup.

Is there anything more to do?

Please enlighten..

JO