[InvalidArgumentException] Package digikraaft/laravel-paystack-subscription has a PHP requirement incompatible with your PHP version, PHP extensions and Composer version

Have a look at this file: https://github.com/digikraaft/laravel-paystack-subscription/blob/v1.0.0/composer.json

This is the "oldest" file he has but it is from June 2020 so this package was developed only for Laravel 7 and lately updated to Laravel 8. It says it all in the file:

 "require": {
        "php": "^7.4",
        "ext-json": "^7.4",
        "digikraaft/paystack-php": "^1.0",
        "dompdf/dompdf": "^0.8.5",
        "guzzlehttp/guzzle": "^6.5",
        "illuminate/contracts": "^7.15",
        "illuminate/database": "^7.15",
        "illuminate/http": "^7.15",
        "illuminate/notifications": "^7.15",
        "illuminate/routing": "^7.15",
        "illuminate/support": "^7.15",
        "illuminate/view": "^7.15",
        "nesbot/carbon": "^2.35",
        "symfony/http-kernel": "^5.1"
    },
    "require-dev": {
        "friendsofphp/php-cs-fixer": "^2.16",
        "orchestra/testbench": "^5.0",
        "phpunit/phpunit": "^9.0",
        "psalm/plugin-laravel": "^1.2",
        "vimeo/psalm": "^3.11"
    },

1. So you are still using older version of php.

2. You do not have ext-json installed

3. Update your composer to version 2

Hope it helps!

How about the other errors?

Paste here the whole error please