Laravel mix helper by default looks for manifest-json file. So if your file is in any other directory or this file doesn't exist then you will see Mix manifest does not exist error. Here you can see three solutions to fix the error.
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Solution 1
Do not use mix in your blade files and you can change
<link rel="stylesheet" href="{{ mix('css/app.css') }}">
To
<link rel="stylesheet" href="{{ asset('css/app.css') }}">
Solution 2
If you don't have root or server access. You can edit the App\Providers\AppServiceProvider file and add the following code to the boot() method.
$this->app->bind('path.public', function() {
return base_path().'/../public_html';
});
Solution 3
If you have root or server access then you need to install the MIX package by using NPM
npm install & npm run dev
Hope this will fix your issue.