Your system tries to create a purchasing info record when saving the PO since it could not find an existing info record for the combination of vendor and material number. But then it failed because the new info record number that it took from the number range was already in the database for table EINA.
You have to analyze table EINA to know which range of numbers is already used, then you need to set your info record number range to a range that will not conflict with the existing numbers