Select top 2 sold products for each month in each department

I want to return for each department, the top 2 sold products of each month, in the year 2016.

My database has the following structure:

Sales(Time_id: Date, Geo_id: char(5), Prod_id: char(6), sales: Int)
Time(Time_id: Date, month: char(2), quarter: char(1), year: char(4))
Geo(Geo_id: char(5), city: varchar(20), dept: char(2), region: char(2))
Product(Prod_id: char(6), class: char(2), category: char(2))

I'm using OracleSQL.

Here's what I've done so far, for some reason I keep getting "not a GROUP BY expression":

SELECT
    t.dep, t.month, t.prod, t.sales
FROM (
    SELECT
        Geo.dept AS dep, Time.month AS month, Sales.Prod_id AS prod, SUM(Sales.sales) AS sales,
        RANK() OVER (PARTITION BY Geo.dept ORDER BY SUM(Sales.sales) DESC NULLS LAST) AS rank
    FROM
        Sales
    INNER JOIN
        Time ON Sales.Time_id = Time.Time_id
    INNER JOIN
        Geo ON Sales.Geo_id = Geo.Geo_id
) t 
WHERE
    t.rank <= 2
GROUP BY
    t.month

As for the returned values, I just want the records to state the department, the month, the product ID and the number of sales.

All non-aggregated columns have to be part of the GROUP BY clause; therefore, as commented it should be moved into the subquery. Also, you forgot to include the year = 2016 condition.

SELECT
    t.dep,
    t.month,
    t.prod,
    t.sales
FROM
    (
        SELECT
            geo.dept         AS dep,
            time.month       AS month,
            sales.prod_id    AS prod,
            SUM(sales.sales) AS sales,
            RANK()
            OVER(PARTITION BY geo.dept
                 ORDER BY
                     SUM(sales.sales) DESC NULLS LAST
            )                AS rank
        FROM
                 sales
            INNER JOIN time ON sales.time_id = time.time_id
            INNER JOIN geo ON sales.geo_id = geo.geo_id
        WHERE
            time.year = 2016
        GROUP BY
            geo.dept,
            time.month,
            sales.prod_id
    ) t
WHERE
    t.rank <= 2