php artisan: "failed to open stream: No such file or directory"

 15

You have done everything correctly except you forget to add the vendor folder into your project directory, just try

 composer i 

Inside the project directory from the terminal, will solve your error. And if you get any other error then remove the composer.lock file and then try again.

UPDATE:

Instead of deleting the composer.lock file when facing issues like PHP version mismatch or dependency version locked just use

composer update