It's not an error message. It's an information window telling you that if you use Nova gestures to bring the notification shade down, it won't be fast because of a bug in Marshmallow that the Nova team has informed Google about.
The Nova team put that there because if people used the Nova gesture to bring the shade down and found it was much slower than swiping from the top edge, people would assume its a problem with Nova, and it's not.