Laravel 5.5 installation error | Whoops, looks like something went wrong

This error showing after installing Laravel 5.5 version that means your composer do not creating env file inside your project folder. make sure there have two types of .env files one file is simple .env file and second file is .env example.

Note: .env file must have key for your project.

so here is two types of solutions: - 
1. If you just want to make current setup work, then run a command 
cp .env.example .env php artisan key:generate

2. You should update your composer if you have an old version. To check your installed version, do so with composer -v.
for composer update run a command
composer self-update

Hopefully you will fix this error , if not please comment below i will reply as soon as possible.