Please help me fill due date column.

JayJoshi 

Since you are looking for a dynamic method, therefore I would like to suggest below shown method to be followed.

How it works:

1. Press Ctrl+F3 to get Range Name dialogue.

      2. Enter an appropriate name in Name box.

      3. Use this formula for references.

=OFFSET(Sheet2!$J$25, 0, 0, COUNTA(Sheet2!$J:$J), 1)

Note:

• Purpose of doing this, if in future you need to add more Task along with days to add then no need to modify the cell references in the formula.
• Repeat step 1 to 3 for "Days to add column", and use this formula.

=OFFSET(Sheet2!$K$25, 0, 0, COUNTA(Sheet2!$K:$K), 1)

Note: Name given to cloumn is ValueList.

• Now enter this formula in cell I25 and fill down.

=IF(ISBLANK(G25),"",H25+VLOOKUP(G25,CHOOSE({1,2}, TaskList, ValueList),2,FALSE))

Adjust cell references in the formula along with Task List & related Days to add, as needed.

**** If this works for you then, you may mark it as best answer as well Like.