Hi,
Please go to PO - Condition tab - click on "Analysis button. Select the condition type NAVS and check the details. I think there is access sequence assigned to NAVS, for which there is no condition record exists.
Go to MEK1 and maintain the condition record for NAVS and check the scenario again with a new PO. Revert back with the screenshots, if not solved.
Regards,
AKPT