It is the second blank entry between the " ".
=IF(MATCH("*"&LEFT(D2,D15)&"*",$Z$Z2:$Z$432,0),AA,D2)
-----------------------------------------------------------------------------------^^^^-----
If syntax is:
=If(Logic_statement, true, false)
If we break down your formula, we get:
If(
MATCH("*"&LEFT(D2,D15)&"*",$Z$Z2:$Z$432,0) //Logic_statement
" ", //True
D2) //False
Have fun!