Heroku Permission Denied

I’ve followed the Heroku guide to the letter, added my API token and ssh user. Stuff.

Each time my build tries to deploy, I’m greeted with a permission denied error.

Initially, since we’re using grunt, I was trying with a deploy script. However, I’ve also tested with a vanilla build option as so:

branch: master
appname: ct-app-development

Can someone advise if there’s an issue with Heroku builds at the moment or whether there’s something else I have to do that’s undocumented.