Bad debt is money you are owed that can't be collected. If you're forecasting for an existing business, you may need to represent a bad debt in your forecast. The best method is to add an expense entry for the amount owed to you. That will figure the value of the debt into your operating costs.
As a supplement, you may want to also add a custom topic to the plan outline, so you can add some notes on the debt and any impacts it's having.