--fake table / data
CREATE TABLE #yourTable
	(user_code char(3))
INSERT INTO #yourTable (user_code)
	select 'A01' union select 'A02' union select 'B01'

--query with inline tally
;WITH E1(N) AS (SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
                SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1),--6
      E2(N) AS (SELECT 1 FROM E1 a, E1 b),--36
cteTally(N) AS (SELECT ROW_NUMBER() OVER (ORDER BY N) FROM E2),
cteAlphaNum AS
	(select N, 
	Value = case when N <= 26 then char(c1.N + 64)
		else cast(N - 27 as varchar) end
	from cteTally c1),
cteFinalList AS
	(select c1.Value + c2.Value + c3.Value as user_code
	from cteAlphaNum c1
	cross join cteAlphaNum c2
	cross join cteAlphaNum c3
	where c1.N <= 26
		and (c2.N <= 26 or c3.N <> 27))
select top 1 fl.user_code
from cteFinalList fl
left join #yourTable t
	on t.user_code = fl.user_code
where t.user_code is null
order by fl.user_code asc