On the Reminder tab shows "Reminders are not supported on this device". The same is true through speech and Sample-Java-Client
Can you please check if you have enabled Timers and Reminders under the Capabilities section for your device? Please follow https://developer.amazon.com/docs/alexa-voice-service/enable-named-timers-and-reminders.html for complete details.
If this does not help fix the issue, can you please add details on the product you are trying to build with details of the request to Alexa Voice Service and response back?