Back around the end of October, Amex Platinum Uber credits started being applied to the Uber Cash balance. Before that the could be used automatically on your next Uber ride. That means that you need to manually select Uber Cash as your payment method in order to use your credits. If you do not have enough to cover the entire ride, you’ll be prompted to add credits to your account, or you can scroll down and select a card to cover just the remaining amount.
The issue with this is that if you don’t specifically request to use your Amex credit by opting to pay with Uber Cash, then you might forget about the credit altogether, and lose it once the month is over. But at least for November, it looks like Uber is rolling over the $15 credit if you failed to use it as per Doctor of Credit. So if you forgot to use your credit last month, check your email, or account to see if they rolled it over. If they did, you should now have a total of $50 to use before the year is over. This is a one time thing, so don;t let your credits expire again.
You can learn more about this credit here.