Topn function using a Parameter

// Careful here: this returns a table
// so it can't be a measure!!! Measures
// never return tables, only scalar
// values.

[5 mejores productos] =
var vProdCount = SELECTEDVALUE( 'Top N'[Top N] )
var vResult =
    TOPN(
        vProdCount,
        ADDCOLUMNS(
            DimProducto[EnglishProductName],
            "@Suma",
                CALCULATE(
                    FactVentas[Ventas]
                )
        ),
        [@Suma]
    )
RETURN 
    vResult

You can try the above... but this is not a measure. The code returns a table (as does yours), so I'm not sure what you're trying to achieve with this. If no selection has been made in the Top N table, then this will return nothing. And for such a selection to occur, the parameter table must be put in a visual. But the code above, as I said, returns a table, so you'll not be able to use it, even if there's been a selection made in the slicer.