Formula to populate cell if source is blank

HI,

I have a formula

=IF($AI2="YES",IF(VLOOKUP($AJ2,Sheet2!$A:$BT,3,0)="X",$D2,""),"")

Currently the returning value is (blank) in cell AK2 but i need to adapt the formula to return the value in E2 if D2 is blank?

Anyone have an idea how?

I've attached the workbook to help.

Regards