Error: 'Output folder doesn't exist and cannot be created': When trying to create a cucumber-nightwatch html report

 MacBook Pro: OS Sierra

IntelliJ 2017.2.6

"cucumber": "^0.10.3",
"nightwatch": "^0.8.18",
"nightwatch-cucumber": "^2.3.3",

I have run this project a number of times and it has created the report OK. But sometimes it starts to fail with the above error. System Settings; password and path variables seems OK and I don't believe I've changed anything 'config' from when it has worked to it not working now. The root folder is not open and I am not trying to edit the html while I run the nightwatch project.

There must be something I (or my code) is changing between it 'working in the morning' and 'failing in the afternoon' but for the life of me I can't think what it is. Any help would be much appreciated.