How do I resolve dependency conflicts in Composer?
I was able to take your composer.json file, run composer update from it without any issues, do an install of Craft from that and update Craft/plugins and install/uninstall Craft/plugins from the Control Panel without any issues.

symfony/yaml v4.1.0 requires php ^7.1.3 -&gt; your PHP version (7.2.14-1+ubuntu18.04.1+deb.sury.org+1) overridden by "config.platform.php" version (7.0) does not satisfy that requirement.


For that one, you've got this in your composer.json file:

  "config": {
    "sort-packages": true,
    "optimize-autoloader": true,
    "platform": {
      "php": "7.0"
    }
  },


Change it to this:

  "config": {
    "sort-packages": true,
    "optimize-autoloader": true,
    "platform": {
      "php": "7.2.14"
    }
  },


And it will go away.