Error on Login/Register: The Mix manifest does not exist.

I have been trying to get a Spark install to work, just at the most basic level. Is there a reliable set of steps anyone knows about? I just get the same error every time I click Login or Register, and I don't know where the breakdown is. I don't see any hints in the install guide about the "Mix Manifest"

ErrorException in helpers.php line 582: The Mix manifest does not exist. (View: /Users/myuser/Developer/myapp/resources/views/vendor/spark/layouts/app.blade.php) (View: /Users/myuser/Developer/myapp/resources/views/vendor/spark/layouts/app.blade.php)

I am using Laravel Valet, so maybe that's the problem?

I have had good luck with most Laravel installs and Quick Start guides, but I can't find a solution to this that has worked yet. I have the following front end stuff.

npm -v
5.0.0

yarn install v0.24.6

Has anyone been stuck here, at the beginning? Thanks in advance for any tips!