Error message "Enter a Printer" when the user clicks on PO out put preview

Dear SRM Gurus,

We are using SRM 5.5 extended classic scenario. When the user presses on Output Preview of PO we are getting a message that "Enter a Printer" and not able to see the preview. This messge we are getting for some particular PO's and for other we are able to see the preview.

Please do the needful..

Thanks and Regards,

Chandra Sekhar.