IFERROR, MATCH, INDEX functions to find differences of two sheets

The problem is you are using & signs incorrectly. What that translates to is:

Using the highlighted row:


=IFERROR(INDEX(BC!$B$2:$B$9480,MATCH(B8&C8,INDEX(BC!$C$2:$C$9480&BC!$D$2:$D$9480,0),0)),"Not Found")

=IFERROR(INDEX(BC!$B$2:$B$9480,MATCH(D123-MMD-C4-EFA-C101_6-00001C04,INDEX(BC!$C$2:$C$9480&BC!$D$2:$D$9480,0),0)),"Not Found")

That full string isn't found on BC anywhere

I will edit this post with a solution in a moment, I just wanted to point that out first, maybe you can figure it out based on that before I do