The forecast is 0 when the budget on the expense item has not been distributed across the months. Distribute the budget, and the item gets a forecast.
How to fix it
Go to Setup and Operation.
Find the expense item and open it by clicking the name or the pencil on the row.
Go to the Cost price estimate tab and click the icon to the right of Remaining rev. Cost price estimate.
Choose Distribute remaining cost price estimate evenly (for you, "cost price estimate" may be called "budget") to distribute the amount evenly over the period, or enter the monthly amounts yourself.
Save.
Good to know
You can paste monthly amounts from Excel instead of typing them in.
You can also describe in words how the cost should be distributed, and let AI accrue it.
Check that the period on the expense item covers the whole time the cost occurs.