Let's make sure to enter all details needed, barb15.
When you check the billable option, that means you want to charge it to your client. We need to make sure that you've selected a customer under Customer/Project column before saving it. You can read this article to know more about billable expenses.
If the same thing happens, we can troubleshoot your browser to see if it is the reason of this error. Start by using an incognito window in converting the purchase order into bill. Also, please clear the cache since it can cause errors in QuickBooks Online. Aside from that, please make sure that your browser is updated or use another one. Old versions can also be the reason of the said issue.
Please comment below if you need further assistance. Wishing you all the best!