Why do I get a warning message that "my bill cannot exceed the scheduled value"?

Each Schedule of Value line has a certain amount in the Contract, when entering Billing for each line you can see all the details, such as Scheduled Value, Prior Completed Amount, and Balance to Finish.

Keep in mind that the amount for This Period Completed Amount cannot exceed the Balance to Finish

If you enter an amount that exceeds the Balance to Finish, you'd get a warning message. You can go back and edit the incorrect Amount and save it again.