What automation are you using to send this notification? Can you please share a screenshot? For example, if using RM to send the notification, please share a screenshot of the specific Rule.
Also, can you just open the Pushover device and send a notification successfully?
Have you followed the directions to set up the Pushover device with your Pushover credentials/keys?
In the Preferences for the Pushover Virtual device you created you put in the API key and User key? At the top of that device you can put a test message in the Device Notification text field and click the Device Notification button to send a test message. If that doesn't work you need to look at the Preferences to make sure you got those all set correctly.