From the comments, you are attempting to populate
@under varchar(50)
with multiple values.
This will not work.
I suggest create a variable table and populate it with the Group_Name values (similar to what you have done in another question).
DECLARE @data TABLE (grp VARCHAR(100));
INSERT INTO @data
select Group_Name
from tbl_AccountGroup
where UnderGroup='Sales A/C'
Use the table to check if @Group_Name exists in the table.
You should be able to figure it out from here.