When inserting the varchar value into the datetime field, it throws conversion error
Since you are not shared the sample data of the Gradd19 value, I suspect the date value is in the yyyy-dd-mm format, so when trying to convert (month value more than 12) it cause the out-of-range value error. 

For sample, if the date format in yyyy-mm-dd, it will execute properly:

DECLARE @FromDate DATETIME;
DECLARE @Gradd19 AS VARCHAR (20) = '2016-11-15';
SELECT @FromDate = CONVERT(VARCHAR, @Gradd19, 105);
SELECT @FromDate;


Incase if it is in yyyy-dd-mm it will throw out-of-range value error

DECLARE @FromDate1 DATETIME;
DECLARE @Gradd191 AS VARCHAR (20) = '2016-15-11';
SELECT @FromDate1 = CONVERT(VARCHAR, @Gradd191, 105); 
SELECT @FromDate1;


So in that case you need do one more conversion to skip the issue, like the below:

DECLARE @FromDate2 DATETIME;
DECLARE @Gradd192 AS VARCHAR (20) = '2016-15-11';
SELECT @FromDate2 = CONVERT(VARCHAR(19), CONVERT(DATETIME, @Gradd192, 105), 120)
SELECT @FromDate2;




So instead of 

CONVERT(VARCHAR, Gradd19, 105) AS ExpiryDate


you need to use

CONVERT(VARCHAR(19), CONVERT(DATETIME, Gradd19, 105), 120) AS ExpiryDate




Update: As you mentioned in the comments the Gradd19 having the values of 20 and 60, it is difficult to filter those non-valid data. But based on the LEN() function you may avoid the non-valid data, then using the CONVERT you can get the VARCAHR field as DATETIME value. 

Based on your sample data the valid date's minimum length is 10 characters, so you can apply the condition in the WHERE clause.

Sample execution:

DECLARE @GradedDETAILS  TABLE (Gradd19 VARCHAR (30));

INSERT INTO @GradedDETAILS (Gradd19) VALUES
('Sep 16 2016 12:00AM'),
('Sep 16 2016 12:00AM'),
('Sep 16 2016 12:00AM'), 
('Sep 19 2016 12:00AM'), 
('Sep 19 2016 12:00AM'), 
('Sep 16 2016 12:00AM'), 
('20'), 
('60'), 
('20-09-2016'), 
('20-09-2016');

SELECT CONVERT(VARCHAR(19), CONVERT(DATETIME, Gradd19, 105), 120)
FROM @GradedDETAILS
WHERE LEN(Gradd19) &gt;= 10


So in your actual query also you need to add the LEN(Gradd19) &gt;= 10 condition and in the SELECT use this to convert as DATETIME by CONVERT(VARCHAR(19), CONVERT(DATETIME, Gradd19, 105), 120)