Setting up Pushover Notifications - "There was an internal error with the Plugin"

I have created an app on with Pushover and entered the user key and app’s api key into the appropriate fields on my project’s alert page. However, when I press the test button I get this message:

“There was an internal error with the Plugin”

Any ideas?

In push over, for my app type I chose Plugin.

I was able to solve the issue by changing from Emergency to High priority.