try:
=QUERY(IMPORTRANGE("URL_or_ID", "Sheet1!E2:E"), "where Col1 is not null", 0)
or:
=IF(A2=1, IMPORTRANGE("URL", "Sheet1!E2:E"&
MAX(IFERROR((IMPORTRANGE("URL", "Sheet1!E2:E")<>"")*(ROW(2:100000))))), )
UPDATE 1:
delete column B and use in B2:
=ARRAYFORMULA(
IF(A2:A=1,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Purchase_Order!E2:E"),
IF(A2:A=2,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "High_Tower!C2:C"),
IF(A2:A=3,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Hotline!D2:D"), ))))
UPDATE 2:
delete column B and use in B2:
=ARRAYFORMULA(
IF(A2:A=1,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Purchase_Order!E"&MAX(
IFERROR((IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Purchase_Order!E2:E")<>"")*(ROW(2:10000))))),
IF(A2:A=2,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "High_Tower!C"&MAX(
IFERROR((IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "High_Tower!C2:C")<>"") *(ROW(2:10000))))),
IF(A2:A=3,IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Hotline!D"&MAX(
IFERROR((IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Hotline!D2:D")<>"") *(ROW(2:10000))))), ))))
UPDATE 3:
use in B2 and drag down:
=IF(A2=1, INDEX(IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Purchase_Order!E2:E"), COUNTIF(A$2:A2, 1)),
IF(A2=2, INDEX(IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "High_Tower!C2:C"), COUNTIF(A$2:A2, 2)),
IF(A2=3, INDEX(IMPORTRANGE("1kkMHXRaaPv0IMZgFofXPROeCNL8OLvyPxoT60gKuryg", "Hotline!D2:D"), COUNTIF(A$2:A2, 3)), )))