Hi,
Put below in S2 and drag below.
=IFERROR("Error - "&INDEX($D$2:$J$2,1,INDEX(MATCH(FALSE,D3:J3=L3:R3,0),,)),"OK-Matched")
or
=IFERROR("Error - "&INDEX($D$2:$J$2,1,SUMPRODUCT(MATCH(FALSE,D4:J4=L4:R4,0))),"OK-Matched")
Both are the same formula just two different non CSE approach.