a) typo - simply missing opening parenthesis before {1,2}

=LOOKUP(9.99E+307,CHOOSE({1,2},0,INDEX(A!$B$2:$M$163,MATCH($C$2,A!$A$2:$A$163,0),MATCH($B3,A!$B$1:$M$1))))
b) I was not referring to DV but to the Named Range definition: RefersTo (not Source)