convert as a CHAR datatype, there's no varchar datatype that you can cast/convert data to:
select CAST(id as CHAR(50)) as col1
select CONVERT(id, CHAR(50)) as colI1
See the following SQL — in action — over at SQL Fiddle:
/*! Build Schema */
create table t9 (id INT, name VARCHAR(55));
insert into t9 (id, name) values (2, 'bob');
/*! SQL Queries */
select CAST(id as CHAR(50)) as col1 from t9;
select CONVERT(id, CHAR(50)) as colI1 from t9;
It seems like you were trying to convert to an incorrect datatype i.e. the syntax that you were using to convert was incorrect.
The convert function takes expr as your column or value:
CONVERT(expr USING transcoding_name)
However, your original query had the syntax backward.