Hi,
Try the below in in Sheet 3 of cell A2 with CTRL + SHIFT + ENTER and then drag it down
IFERROR(INDEX(Arr_1,MATCH(0,COUNTIF($A$1:A1,Arr_1),0)),INDEX(Arr_2,MATCH(0,COUNTIF($A$1:A1,Arr_2),0)))
Define name as below
Arr_1 = Sheet1!$A$1:$A$21
Arr_2 = Sheet2!$A$1:$A$15
See the attached for ref.