Hello Ankit,
DECLARE @SELECT VARCHAR(MAX) = (SELECT UserCode + '_EmailAddressInfo' FROM SUser_Master)
Here you are trying to store result of the Query to a variable @SELECT.
But what if this Query returns more than one value? SQL got confused that which value it should store?
that's why error says "SubQuery Returns More Than One Value."
Here You Have some options...
Either you put specific where condition that returns exact one value...
Or Take top 1 like "Select top 1 userCode from SUser_Master".(Last choice for me).
Or Concat the Result (using ',' or '|' or etc.) and store that into your variable.
Hope You have understand the issue and this might help you.