billingCodeID or departmentID is a Required Field Error

If an Autotask project task is missing a value for certain fields, when you update the task's status from TopLeft you might see one of the following error messages:

Or

Autotask only requires tasks to have these fields provided when the task has a primary or secondary resource assigned.

To solve this error, simply set the values for those tasks in TopLeft. Then try your change again.