Mukhter26
Firstly, Welcome to the Chandoo.org Forums
Your formula =IF(A131="","",LOOKUP(A131,'Inventory Master'!$B$4:$B$404,'Inventory Master'!$D$4:$D$404))
says if A131= nothing put nothing
If A131 <> nothing lookup A131 in the range: 'Inventory Master'!$B$4:$B$404, and retrieve the matching value from range: 'Inventory Master'!$D$4:$D$404)
Both Ranges are the same length 4:404
If it isn't working It is either because the value in A131 isn't in the range 'Inventory Master'!$B$4:$B$404
Ensure that both A131 and range: 'Inventory Master'!$B$4:$B$404 don't have leading or trailing spaces eg: "A" <> "A "
You could also try a newer:
=IF(A131="","",INDEX('Inventory Master'!$D$4:$D$404,MATCH(A131,'Inventory Master'!$B$4:$B$404,0)))
If the above doesn't help can you post the values you have in each range or a sample file