When I click on maximum btc(57$) ,the fee shows 11.86$ and with same network fee when I want to send 50$ then I have to pay 15$ fee.
hd49728 is correct. The transaction fee doesn't depend on the amount you send.
As already said, the transaction fee depends on number of inputs and outputs.
In the left image, you are sending the whole balance and your transaction would have 1 output.
In the right image, you are not sending the whole balance and your transaction would have 2 outputs. The extra output which would be for receiving the change makes the transaction larger and causes the fee to be higher.