When you got your initial count you likely ran the following:

SELECT COUNT(Product_Purchased) Num, Product_Purchased
FROM Data
GROUP BY Product_Purchased

All of this must be contained within your subquery, but you, perhaps by accident, put your GROUP BY in the main query. Instead:

SELECT CONCAT('Product_Purchased', '-', 'No') Total_Product_Purchased
FROM
(
    SELECT COUNT(Product_Purchased) Num, Product_Purchased
    FROM Data
    GROUP BY Product_Purchased
) A;

This will no longer throw an error, but you still have a problem in that you are attempting to concatenate string-literals instead of columns returned in the subquery's result set. Instead:

SELECT CONCAT(Product_Purchased, '-', Num) Total_Product_Purchased
FROM
(
    SELECT COUNT(Product_Purchased) Num, Product_Purchased
    FROM Data
    GROUP BY Product_Purchased
) A;

Furthermore, the subquery is superfluous as you can do this all in the same query:

SELECT CONCAT(Product_Purchased, '-', COUNT(Product_Purchased)) Total_Product_Purchased
FROM Data
GROUP BY Product_Purchased