Unable to set up plugin

Hi, I am trying to set up this plugin to send push notifications about posts to my firebase project topics. However, I am not entirely sure how to set up the plugin as I cannot find any clear instructions on how to get data such as the API key.

I believe this (the API key) to be the most likely cause of the issue, which is that when I test the plugin, nothing happens. I This is the response I get when I click the “Click here to send a test notification” button: {“headers”:{},”body”:”\n\n.

Any help would be much appreciated, I have currently entered the contents of the “private_key” field of the json file which I got from the service accounts tab of my firebase project.