I assume you are referring to the Billable expenses account balance in the Summary. Click on the blue balance figure to drill down and see what contributes to it. Make sure everything has been billed out.
Another, perhaps easier approach–depending on how many customers and expenses you’ve had, is to go to the Customers tab and look for a customer who has a balance in the Uninvoiced items column. Drill down on that balance and see what hasn’t been invoiced. This is explained here:
https://forum.manager.io/t/tracking-billable-expenses/5503
The transaction(s) causing the balance to be greater than zero should be easy to find.