For the below once I have a date in [In Progress Date] or [Actual Completion Date] it sets #Invalid Operation when I expect a date result.
=IF([Actual Completion Date]@row = 0, IF([In Progress Date]@row <> 0, [In Progress Date]@row + ([Estimated Days to Completion]@row * [Resource Time Percentage]@row), "TBD"), [Actual Completion Date]@row)