You problems my way were

1) you were including the header
2) you'd moved the countif from the results

Block
Main Building
Block A
Block B
BSS House


Is what i get, just changing the countif part to be column D, you were counting if your column a was in column a

=INDEX($A$1:$A$20, MATCH(0, COUNTIF($D$1:D1, $A$1:$A$20), 0))