Unfortunately, there are countless reasons why your dependencies may not be installing properly. A common root relates to an ancient version of Node (node -v) and npm (npm -v) installed. As a first step, visit nodejs.org and update those.
Otherwise, often, it's related to a faulty lock file that needs to be deleted. Give this series of commands a try to install everything from scratch:
rm -rf node_modules
rm package-lock.json yarn.lock
npm cache clear --force
npm installIf you come across a failure message like this one...
These dependencies were not found:
* /Users/you/Sites/folder/resources/js/app.js...then you're likely using npm 5.2 (npm -v). This version introduced a bug that caused installation errors for Mix. The issue has been fixed as of npm 5.3. Please upgrade, and then reinstall your package.json dependencies from scratch:
rm -rf node_modules
rm package-lock.json yarn.lock
npm cache clear --force
npm install