composer unable to install laravel/passport

This link may help you https://github.com/laravel/passport/issues/774

They said, it fixed their issue

composer require paragonie/random_compat:2.*
composer require laravel/passport

Same issue with Laravel 5.5.40 or higher. So you need to specify the version of the passport like this;

composer require paragonie/random_compat:2.*
composer require laravel/passport=~4.0