You don't need to enter the private key when sending the fund to coinbases. Leave "Description" empty. The private key should be entered when you create the wallet.

Create a new wallet.
Select "Import bitcoin addresses or private keys" and click on "Next". Enter your private key and click on "Next". After that, you should see your balance.

Go "Send" tab. Enter your coinbase address in "Pay to" field and the amount of bitcoin you want to send in "Amount" field.
You can click on "Max" button if you want to send the entire balance.

A new window where you should select the fee.
Select the fee and click on "Next".

Note that the higher fee you pay, the sooner your transaction will be confirmed.