The reason why your formula is returning nothing is because the formula contains an error.
=IFERROR(formula,"") will return nothing if the formula results in an error.
=IFERROR(formula,"popcorn") will return popcorn if the formula results in an error.
You have to find out why INDEX('Usable Data'!$B$2:$M$201,SMALL(IF('UsableData'!$M$2:$M$201='Working Staff'!$D$1,ROW('UsableData'!$M$2:$M$201)-1),ROW(A1)),3) is giving you an error. This should be an array formula. Without looking too much into it, try =INDEX('Usable Data'!$B$2:$M$201,SMALL(IF('UsableData'!$M$2:$M$201='Working Staff'!$D$1,ROW('UsableData'!$M$2:$M$201)-1),ROW(A1)),3) Ctrl Shift Enter instead of just Enter.