Initially, you need this formula in the Status column:
=IF(A2>$F$1,"not started yet",IF(B2<$F$1,"finished","SelectStatus"))
If the two conditions failed, the formula will show (SelectStatus) which will be the source reference of the drop-down list in the data validation.
You will notice that when the cell contains (SelectStatus), the drop-down list will be available.
One restriction to this solution is that when you select the status from the drop-down list, you can't select it again until you apply the formula on the range again or type (SelectStatus) in the cell manually!