Open the Alexa app, tap the devices icon at the bottom.
Then tap Cameras near the top (you may have to scroll right/left to see it).
Now that you're in the Cameras section, look for your Ring doorbell. Tap your Ring doorbell and the live view will come on. Tap the gear icon in the upper-right corner to enter Settings. In Settings, you can enable Motion announcements and Doorbell press announcements. Make sure you have chosen which Echo devices that you want to hear the announcements. If needed, tap Announcement Devices to choose your Echo devices. On the same page, scroll all the way down to make sure that the Enabled toggle is actually enabled.
See attached screenshot for an example.