My guess is the funds have dried up already for this year.

The first 2 million thank-yous given will include $5 to the delivery driver at no cost to the customer. After that, customers can continue to thank their driver via the feature all year round, and drivers are able to see the number of times they are thanked within their app.

https://www.aboutamazon.com/news/transportation/alexa-thank-my-driver-feature