[RuntimeException] No supported encrypter found. The cipher and / or key length are invalid.

[chotosite]

When I try php artisan serve it says [RuntimeException] No supported encrypter found. The cipher and / or key length are invalid.

I also tried:
php artisan key:generate
composer update
composer install

But nothing is working

I have installed laravel 6.x. If I run laravel new blog everything is working fine.

[qlint]

This project was built in Laravel 5. I'm not sure if Laravel is backward compatible - maybe that is where your problem is. What output do you get on composer install?

[chotosite]

This is the output of composer install:
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Nothing to install or update
Package phpoffice/phpexcel is abandoned, you should avoid using it. Use phpoffice/phpspreadsheet instead.
Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.
Generating autoload files
Carbon 1 is deprecated, see how to migrate to Carbon 2.
https://carbon.nesbot.com/docs/#api-carbon-2
You can run './vendor/bin/upgrade-carbon' to get help in updating carbon and other frameworks and libraries that depend on it.

Illuminate\Foundation\ComposerScripts::postInstall
php artisan optimize

[RuntimeException]
No supported encrypter found. The cipher and / or key length are invalid.

Script php artisan optimize handling the post-install-cmd event returned with error code 1

[hamza-gaya]

Nothing worked cipher is error is appearing in cmd of windows while entering the command php artisan serve

[abdelsalamshahlol]

This problem can be solved by doing the following:

1- Create .env in the root folder of the project and paste this placeholder code

APP_NAME=Laravel
APP_ENV=local
APP_KEY=base64:ST+O4okIRa/lJsN9f3Y29ggrCIeXC4dYyv5yDp7Zvl4=
APP_DEBUG=true
APP_URL=http://localhost

LOG_CHANNEL=stack

DB_CONNECTION=mysql
DB_HOST=127.0.0.1
DB_PORT=3306
DB_DATABASE=database
DB_USERNAME=root
DB_PASSWORD=

2 - Run php artisan key:generate in the console
3 - Install project dependencies composer install

[nil-borad]

**Note ** Delete Vendor Folder From cloned directory to be sure

1. Copy Env File From Another Application (make necessary changes)
2. php artisan key:generate
3. composer install