Try................
1] In "Sheet1 - Index no" R3, copied down :
=IF(S3=FALSE,"",MAX(R$2:R2)+1)
2] In "Sheet1 - Selected" T3, copied down :
=IFERROR(INDEX($B$3:$B$46,MATCH(ROWS(A$1:A1),$R$3:$R$46,0)),"")
3] Delete all formulas in "Sheet1" Column U
4] Define Name >> Name : Stitch
>> Refer to: =OFFSET(Sheet1!$T$3,0,0,COUNTIF(Sheet1!$T:$T,"?*")-1,1)
>> OK
5] In "Sheet 2" B5 >> Data Validation >> Setting
>> Allow : List
>> Sources : =Stitch
>>OK
Regards
Bosco