Your formula does not return an error when the cell is empty, it is the same as 0
Perhaps like this ?

=IFERROR(IF((HLOOKUP(A2,years!A$1:F$5,2,0))=0,"",H LOOKUP(A2,years!A$1:F$5,2;0)),"")


( the space between H and LOOKUP is added by the board. Disregard it)

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