Could not find package with stability dev in a version installable using your PHP version
The package that triggers the error is  drupalcommerce/project-base. If you check the composer information for that package you see: 

  "require": {
        "php": "&gt;=7.0.8",
        "ext-curl": "*",
        "composer/installers": "^1.2",
        "cweagans/composer-patches": "^1.6.5",
        [...]


As you see, it requires PHP 7.0.8 or higher. You have PHP 5.6.40. You should upgrade your PHP, not only because this package but also because PHP 7.x is much faster and PHP 5.6 is not recommended anymore, not to forget that PHP 5.6 end of life was hit a year ago, on the beginning of 2019.