Since you need a varchar
value in the same field together with int/bool, you need to make sure every row of that field has the same data type.
Isnull(Convert(varchar(50), ReturnedValue), 'UnRegistered') AS ReturnedValue
Or you can use a CASE
as
Case when ReturnedValue is null then 'UnRegistered'
else convert(varchar(50), ReturnedValue) end as ReturnedValue