Your Composer dependencies require a PHP version ">= 7.3.0". You are running 5.6.40. code example
Example 1: Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 7.3.0".
composer install --ignore-platform-reqs
Example 2: your composer dependencies require a php version ">= 7.3.0"
Project or global composer.json file
{
"config": {
"platform-check": false
}
}