When you clicked on Tether USD (USDT) you are then taken to this next window where you get an overview of the withdraw form.  You can see the selected USDT in the form already in place and the button labeled WITHDRAW is blackened to indicate you are initiating a withdraw.

Demonstration: Kraken.com > Funding > Withdraw [form overview]

You will move to the field labeled "Withdraw Type". It has an arrow to the right allowing you to click it for a drop down menu.

Click the arrow to open the drop down menu and select "Tether USD (TRC20)"

Demonstration: Kraken.com > Funding > Withdraw > Type > Tether USD (TRC20)

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