I have installed rbenv on an Ubuntu sever. I can cd into my application directory and run $ bundle without issue, however I need to run $ sudo bundle exec
Why you get the error has been addressed already. But I was able to get around it by saying:
sudo /full/path/to/bundle exec ...
In my case, I'm using rbenv, so I had to:
sudo /home/renier/.rbenv/shims/bundle exec ...
That worked. To get sudo to not ask for a password, you would need to configure your /etc/sudoers file for this. See https://serverfault.com/a/160587.