You will need to bump your Laravel Mix, SASS Loader dependencies to the latest and install PostCSS and Webpack.

npm i --save-dev laravel-mix@latest && 
npm i --save-dev sass-loader@latest && 
npm i --save-dev postcss@latest && 
npm i --save-dev webpack@latest

You should now have Laravel Mix version 6, so the only remaining step is to compile your assets. Then, use the new Mix executable or use npm.

npx mix -p

OR

npm run prod

package.json

{
    "private": true,
    "scripts": {
        "dev": "npm run development",
        "development": "mix",
        "watch": "mix watch",
        "watch-poll": "mix watch -- --watch-options-poll=1000",
        "hot": "mix watch --hot",
        "prod": "npm run production",
        "production": "mix --production"
    },
    "devDependencies": {
        "axios": "^0.21.1",
        "bootstrap": "^4.0.0",
        "bootstrap-vue": "^2.21.2",
        "clean-webpack-plugin": "^3.0.0",
        "cross-env": "^5.1",
        "jquery": "^3.2",
        "laravel-mix": "^6.0.9",
        "less": "^3.12.2",
        "less-loader": "^7.0.0",
        "lodash": "^4.17.13",
        "popper.js": "^1.12",
        "postcss": "^8.2.4",
        "resolve-url-loader": "^3.1.2",
        "sass": "^1.20.1",
        "sass-loader": "^10.1.1",
        "uglify-js": "^3.10.3",
        "vue": "^2.5.17",
        "vue-template-compiler": "^2.6.10",
        "webpack": "^5.14.0"
    }
}

In case somebody has a similar issue.

NPM is moody. Sometimes it's worth to try deleting all modules and install them again:

on Linux it's a simple command: rm -rf node_modules in a project root folder,

than just run npm install again.

Recently laravel mix has been update to laravel mix 6 so I would advise to check the current version of npm/node you are using and the try to upgrade it to the latest version.

npm -v

You can upgrade to the latest version of npm using:

npm install -g npm@latest

Or upgrade to the most recent release:

npm install -g npm@next

I tried many solutions but only this worked for me:

First, update your laravel-mix

npm install laravel-mix@latest

Then do a clean npm install:

npm clean-install

that should fix the laravel 8 mix error.

I had the same issue (Win 10 as host and Homestead in Vagrant). I tried the above but still got the error. The following ended up working for me:

1. open up powershell as admin
2. navigate to project root
3. run this command

node c:\<path to node installation>\node_modules\npm\bin\npm-cli.js install --scripts-prepend-node-path=auto

In previous versions installing node packages from the vm into the mounted drive (with no bin links because windows) was okay, laravel mix would work. My guess is because the package.json file was more verbose and specified full paths - difference in package.json files here.

With the terse syntax in the new version it needs the bin link to follow the reference. Running the command without admin privileges failed for me because renaming files, might be different for you. Going forward this will need to be the new process for installing node packages (on host vs on vm).

I had the same problem after installing laravel/ui:

composer require laravel/ui
php artisan ui vue --auth

In this case there are two possible solutions:

1. Simply delete the node_modules folder with RMDIR "FOLDERNAME" /S /Q and reinstall it with npm install
2. run npm install only after laravel/ui and ui vue has installed (i know, sounds crazy, but that's exactly how it was for me)

After you have done one of the two points, do npm run dev.

npm install laravel-mix@latest

and you may need to install vue-loader legacy peer dependencies. Generally it gets installed or updated automatically during npm run dev

Best method to fix mix not found error is to reinstall npm using the command:
npm clean-install

I faced the same situation. After couple of hours finally got it working. What I did:

1. Updated NPM npm install -g npm@latest
2. Updated laravel-mix (6.0.31 at the moment of writing)
3. Updated Vagrantfile https://laravel.com/docs/8.x/homestead#symbolic-links-on-windows
4. Reload PC (vagrant reload --provision would probably be enough)
5. Run terminal/phpstorm as admin
6. Remove old node_modules, yarn.lock, package.lock, clear NPM cache
7. IMPORTANT Use YARN! sudo yarn install then sudo yarn run dev

please run this command . Its works for me.

yarn install --ignore-engines