So, I think we should mark it as solved, and change the title to “Netlify build error when yarn.lock file missing” 
I am in the habit of submitting my yarn.lock, so I did not think of it right away until I saw your build command.
So, I think we should mark it as solved, and change the title to “Netlify build error when yarn.lock file missing” I am in the habit of submitting my yarn.lock, so I did not think of it right away until I saw your build command.