going by your workbook and if you have a modern enough Excel, I'm guessing the following formula does the trick.
[C2]=XLOOKUP(A2,Key!A:A,Key!B:B,NA(),0,-1)
for older, not too old versions, maybe this
[C2]=INDEX(Key!B:B,AGGREGATE(14,6,ROW(Key!A:A)/(Key!A:A=Expected!A2),1))
Did use lazy references, adapt to your situation as it is not really advised to reference full columns. Just shows the technique.