Can you briefly explain me what all the steps you have followed for PO creation?
“Pending or posted invoices exist”, means PO might have pending invoiced … Have a look in the Pro-forma invoice > Packing slip or Proforma invoice > Invoice. you may invoice which is incomplete.
the particular purchase order may have the pending invoice in it. if you want to check whether pending invoice is there or not goto that particular po>> inquires>> pending invoices. if it is there than first you need to go po>> posting>> invoice>> it may have error with warning delete that invoice and close this form. now go back to posting and invoice it.