Issue on installing Passport package in Laravel 5.5
You are getting this error because you are trying to install an old version of passport into the latest version of Laravel. In order to install passport using composer First thing first, remove the "laravel/passport": "^1.0" from your composer.json your composer json should look like this.
"require": { "php": ">=7.0.0", "fideloper/proxy": "~3.3", "laravel/framework": "5.5.*", "laravel/tinker": "~1.0", "laravelcollective/html": "^5.2.0",}and then Follow the steps that are given in docs
Try to install passport using composer require laravel/passport as it will automatically install the latest version of passport for your Laravel 5.5 but make sure you haven't included passport in require section of your composer.json.
Please use
"php": ">=7.0.0", "fideloper/proxy": "~3.3", "laravel/framework": "5.5.*", "laravel/passport": "^4.0", "laravel/tinker": "~1.0"