When selling shares, you need to enter the amount of shares you want to sell (and not the $ amount). This is to prevent you having incorrect orders being placed, if the market moves before your order is executed.

For example, if you have $10,000 of Apple and you want to sell all of them, if the price of Apple goes down, you will have less than $10,000. If it goes up, you will have more than $10,000 to sell.

In the first instance, your order will not be processed as you have less than $10,000 to sell. In the case of the price moving up, when you sell $10,000 worth, you will be left with a small amount of stock which would not be ideal as you will need to pay brokerage again to sell out your position. For this reason, selling is done by the number of shares you own (including fractional amounts).