Basic installation configuration missing

I’ve just followed the instructions:

composer create-project --prefer-dist yiisoft/yii2-app-basic basic

After that I’ve just ran the composer install command.

When I open my webpage I am getting:

yii\web\Request::cookieValidationKey must be configured with a secret key.

I tried to search for this on web, and I have no config/main.php.

What do I miss?

I am using my own nginx/php,

I have just used the exact command and - composer install is done automatically and cookieValidationKey is set automatically (in config/web.php) by postInstall step. So there must be something wrong with the way your composer works.