Hello again, @abricoetzer. I appreciate you following the recommended steps provided by my colleagues above. I have another solution for you to help fix this issue. 

Let's check the billable accounts that we set up and make it assigned to a single account. Here's how:

1. Click the Gear icon at the top. 
2. Select Account and Settings.
3. In the left menu, hit Expenses. 
4. Go to the Bills and expenses section and click the pencil icon. 
5. Choose In a single account, then Save. 
6. Hit Done. 

Once finished, try to create a Purchase Order again and check if it works. 

Get back to me if you have any other questions by leaving a comment below. I'm more than happy to further assist you. Have a great day!