You are refererencing column C when you should be referencing column D...

=IF(B3<>"",STDEV(IF($D$3:$D$15=D3,$F$3:$F$15)),"")

confirmed with CTRL+SHIFT+ENTER not just ENTER.

and D3 should be:

=LOOKUP(REPT("z",255),B$3:B3)

copied down.