Adding a percent column to MS Access Query

I'm trying to add a column which calculates percentages of different products in MS Access Query. Basically, this is the structure of the query that I'm trying to reach:

Product | Total | Percentage
Prod1    |   15    |    21.13%
Prod2    |   23    |    32.39%
Prod3    |   33    |    46.48%
Product |   71    |    100%

The formula for finding the percent I use is: ([Total Q of a Product]/[Totals of all Products])*100, but when I try to use the expression builder (since my SQL skills are basic) in MS Access to calculate it..

= [CountOfProcuts] / Sum([CountOfProducts])

..I receive an error message "Cannot have aggregate function in GROUP BY clause.. (and the expression goes here)". I also tried the option with two queries: one that calculates only the totals and another that use the first one to calculate the percentages, but the result was the same.

I'll be grateful if someone can help me with this.

You can get all but the last row of your desired output with this query.

SELECT
    y.Product,
    y.Total,
    Format((y.Total/sub.SumOfTotal),'#.##%') AS Percentage
FROM
    YourTable AS y,
    (
        SELECT Sum(Total) AS SumOfTotal
        FROM YourTable
    ) AS sub;

Since that query does not include a JOIN or WHERE condition, it returns a cross join between the table and the single row of the subquery.

If you need the last row from your question example, you can UNION the query with another which returns the fabricated row you want. In this example, I used a custom Dual table which is designed to always contain one and only one row. But you could substitute another table or query which returns a single row.

SELECT
    y.Product,
    y.Total,
    Format((y.Total/sub.SumOfTotal),'#.##%') AS Percentage
FROM
    YourTable AS y,
    (
        SELECT Sum(Total) AS SumOfTotal
        FROM YourTable
    ) AS sub
UNION ALL
SELECT
    'Product',
    DSum('Total', 'YourTable'),
    '100%'
FROM Dual;