Error in index match function

Hi and welcome to the forum

why cant you attach your file?

Try

=INDEX(IF(Worksheet!C1="Y",Plan!C:C,Plan!D:D),MATCH(1,(Worksheet!C3=Plan!B:B)*(Worksheet!A3=Plan!A:A),0))

The index/match combo is fairly easy once you understand the syntax required. It is actually 2 formulas (well, 3 if you consider 2 match()'s) combined.

=INDEX(range,row,column)

=index(range-to-look-in,row-number-to-use,column-number-to-use) It will return the value at the intersection of the row and column

Thats fine if you happen to know the row and column you want to use, but more often than not, at least 1 of those is not known. So this is where the match() comes in.
=MATCH(criteria,range,0) 0=exact match, 1 = less than, -1 = greater than criteria
=MATCH("TEST",A1:A10,0) This will search for an exact match of the word TEST in the range A1:A10, and tell you which row it was found in
that takes care of finding the row...finding the column is exactly the same...
=match("Jan",A1:J1,0) This will search for Jan in the range A1:J10

So when you combine them all, you get...

=index(A1:J10,match("Test",A1:A10,0),match("Jan"A1:J1,0))

...search for a cell in column A containing TEST - search for a column titled Jan in row 1...give me the contents of the intersection

Hope that helped some?