Can I trade in my iPhone 16 for an iPhone 16 Pro in US?

If you bought it 14 days or less ago, you can return it and get the phone you want. If you've owned it longer than 14 days, as Zachyy correctly suggests, you can't trade it in currently. You could, if you want, sell it to someone and use the money towards the phone you want.

Apple does not allow you to trade in the most recent lineup of iPhones. Currently you can only trade in the iPhone 15 lineup and earlier. When the next lineup of iPhones are released, you will then be able to trade in your iPhone 16.

In what country did you buy the iPhone 16?

Then answer is a solid no, period.