We appreciate you for including the exact error message you encountered in this post. Let me provide some steps to solve the issue you're having in QuickBooks Online (QBO), FlyHouse.
When you enable the setting for Track Expenses and items by customer and select Make expenses and items billable, you'll have a section to choose one of two options for the Billable expense income account. One of the possible reasons why this error occurs is that you've selected the second option for multiple accounts and it wasn't set up properly.
That said, you're on the right track for selecting the In a single account to track your expenses and for checking the use for billable expenses option.


To troubleshoot the issue, just follow the steps below:
- Go to the Accounts and Settings.
- Select the Advanced tab, then click on the Chart of Accounts section.
- Choose your billable expense income account.
- Even if this account looks accurate, it may be helpful to try to select a different account, and then Save it.
- Edit it again, and then reselect the original account to kickstart the process.

If the error continues after performing the steps above, I suggest logging into your account using a private browser/ incognito mode. Private browsing prevents cookies from being stored on your computer, making it a great place to identify browser issues. Use these keyboard shortcuts:
- Ctrl+ Shift + N for Google Chrome
- Ctrl + Shift + P for Mozilla Firefox and Microsoft Edge
- Command + Shift + N for Safari
If it works, return to your default browser and clear its cache. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system, and you'll be able to work with a clean slate. Switching to another compatible one to help rectify the issue.
Additionally, I'll add this article as your reference for receiving and categorizing invoice payments in QuickBooks Online: Record invoice payments in QuickBooks Online.
Please let me know how these steps work out and if you have any follow-up questions. I'll be here to help you out. You have a good one.