Hi,
I will try to explain the flow.
When you create a new document it will get the related No. Series (in your case Receiving No. Series) from setup (Purchases & Payables Setup in your case) table and put it in the field Receiving No. Series field. If there is no No. Series setup then on creation it will not give any error but when you try to post/receive it then it will check for No. series and to get a No from that No. series and on finding this field empty will throw an error.
In your case I think what happens is that when this order is created there is no Receiving No series setup in the Purchases & Payables setup and now when you try to post/receive it checks Receiving No. series field value and give error. Now even if you fill the value in Purchases & Payable setup it will not be updated in already created Purchase Order.
Check if you have filled Receiving No series in Purchases & Payables then create a new Purchase Order because from user end you might not be able to update this field's value. But if you access to development environment then run page 38 Purchase Header, go to that specific record and put value in Receiving No series.