Tron (TRX )and other crypto currencies are traded through exchange where usually USD are not acceptable. How ever you can still buy it but out will need few more steps.
First you need to join CoinBase. You can add payment method and after buy ETH currency thorough your card with USD or any currency. Transfer your ETH to www.binance.com.
You could use GDX from where you could deposit from coinbase and after withdrawal to www.binance.com. It will save transfer fee.
After withdraw of ETH to Binance then buy TRX through your ETH fund in Binance.