It seems that there is no lib-file created in your project. A work around may be to create a project in 6.8.1 (or earlier) and copy the lib folder to the sass folder in your own project (Projectfolder > theme >styles > sass).
The warnings are totally normal, so you can ignore this.
Can you tell me how you have structured your project theme?
In your project you should have:
That is the setup I have and how I structured ux-theming when we created this. In the top of your Gulpfile.js it will say what folders it will monitor (sourceStyleFolder).
If I am correct, it will only pickup on changes in files that are already there. So if you create a new file, you will have to restart gulp.
If it doesn't work, please file an issue on my Github repository. Please tell me which node version (type `node -v` in your terminal) and gulp version (gulp -v) you are using. I'll see if I can reproduce this issue.
Thanks for your answer. The folder tree is exactly the same as in my tree, but that is the wrong one. I filled an issue yesterday which has been solved today. the lib folder should be created automatically under the sass folder when creating a new app.