Can you please provide the CREATE TABLE statement of the h table you are referring to, as well as some sample data?

Edit: A small test to get you started anyway. It's probably because of the extra space in your pattern. Remove the '123 456' row from the @test-2 table and see the difference...

declare @test-2 table (po_number nvarchar(50))

insert into @test-2 (po_number)

values ('123 456'),('not a po'),('123456'),('123-456'),('not a po'),('-123456')

select

CASE WHEN PATINDEX('%[^0-9 ]%',po_number) = 0

THEN Cast(RTrim(LTrim(po_number)) as BigInt)

ELSE 0

END

from @test-2