You will need to convert your USD into USDT so that you can move them through TRC20.
To do this you will need to go the Trade section of Kraken.com
On the Trade page you will need to be sure you are in the correct market. Under "Market" you will see a paring. You can click on the pairing or the magnifying glass to the right of it.
A window will pop up so that you can find and select the pairing you are need to work with.
Look for "USDT / USD" and click on the button labeled "Trade" to confirm that is the pairing you are working with.
The pop up will go away and you will be returned to the page where you can now see the pairing market you selected under the word "Market"
By default the page will show you a section labeled "Order" with "Buy/Sell" buttons under it. This is the form you will use to tell Kraken you want to use your USD to buy some USDT.
The "Volume" box by default shows "USDT". We will need to change that to "USD".
After you have made this change for the "Volume" box, you will notice the "Total" box will change from USD into USDT automatically.
Now that you have things setup properly, you can enter the values you want to work with.
Example: Let's say you have $1000 in USD and you want to use it all to get USDT, then you would enter 1000.00 in the "Volume" box ; or equally, you can enter 1000.00 in the USDT "Total" box and the site will auto-calculate and auto-populate the "Volume" box for you.
After you have entered your values you can then click the "Buy" button.
The whole purpose here is to basically convert your USD into a crypto format: USDT. (USDT is also known as USD Tether.)
After you have clicked the "Buy" button your order will show up below, under "Open Orders". You can keep track of its progression under the "Status" column.
Only after the order has been successfully filled can you move onto the next section of withdrawing your USDT through TRC20.