You will need to cast or convert as a CHAR datatype, there is no varchar datatype that you can cast/convert data to:

select CAST(id as CHAR(50)) as col1 
from t9;

select CONVERT(id, CHAR(50)) as colI1 
from t9;

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;

Besides the fact that you were trying to convert to an incorrect datatype, the syntax that you were using for convert was incorrect. The convert function uses the following where expr is your column or value:

 CONVERT(expr,type)

or

 CONVERT(expr USING transcoding_name)

Your original query had the syntax backwards.

Thanks to Ergest Basha who answered this in a comment:

Here is the result:

DROP TABLE dbo.TEST_FLOAT;
CREATE TABLE dbo.TEST_FLOAT (
    AMOUNT    FLOAT
);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (11546.25);
SELECT AMOUNT, 
       CAST(AMOUNT AS VARCHAR(10)) AS AMOUNT_AS_CHAR,
       CONVERT (VARCHAR(10), AMOUNT, 128) AS AMOUNT_AS_CHAR2
FROM dbo.TEST_FLOAT

This is now working in AMOUNT_AS_CHAR2

UPDATE

I just discovered this solution DOES NOT WORK when the amount is ZERO. My work around is as follows:

DROP TABLE dbo.TEST_FLOAT;
CREATE TABLE dbo.TEST_FLOAT (
    AMOUNT    FLOAT
);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (11546.25);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (0.0);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (-0.1);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (2.0);
INSERT INTO dbo.TEST_FLOAT (AMOUNT) VALUES (NULL);

SELECT AMOUNT, 
       CAST(AMOUNT AS VARCHAR(10)) AS AMOUNT_AS_CHAR,
       CONVERT(VARCHAR(10), AMOUNT, 128) AS AMOUNT_AS_CHAR2,
       CASE [AMOUNT] WHEN 0 THEN '0' ELSE CONVERT(VARCHAR(59), [AMOUNT], 128) END AS AMOUNT_AS_CHAR3
FROM dbo.TEST_FLOAT

Adding the INLINE CASE wrapper gets around the issue...