Hi there, ASH_T123, It's good to see you here in the Community.

Let me get the help you need concerning Purchase Order.

In your case, upon deleting the Bill, maybe the Bill being deleted is not the one linked to the Purchase Order.

To further isolate the issue, you can check the Purchase Order and verify the specific Bill linked to it.

Here's what you'll need to do:

1. Click Expenses in the left navigation menu.
2. Go to Vendors tab, then click the name of the specific vendor.
3. In the Vendor profile page, go to the Transaction List tab and locate the Purchase Order.
4. In the Purchase Order window, click 1 linked transaction link found below the Vendor's name.
5. Click Bill link.

From there, you'd able to know the Bill link to that Purchase Order.

Moreover, in order to make the Purchase Order open again, make sure to delete the correct Bill link to it.

Additionally, I wanted to provide this article for more information: Apply a purchase order to a vendor transaction.

If you have further questions in managing your transactions, please feel free to visit the Community again, Have a great day!