I don't understand why I have to pay additional tax for Gross proceeds less commission for my form 1099-b when I already paid taxes for all these

How did you already pay the tax?

And you don't pay tax on the proceeds.  You have to enter the sales price and your cost.  You only pay tax on the GAIN.  Or you get to deduct a loss.  Be sure to add any reinvested dividends to your cost. 

If any taxes were withheld, or paid by estimated tax, be sure those are recorded.