Composerise Drupal command syntax and starting location
composer-root should not be your Home directory. It looks like you might have assumed that because there is a directory named git there, but that's not what the installation instructions at https://github.com/grasmash/composerize-drupal are referring to. Those instructions assume that your project files are being tracked as part of a Git repository. You can tell where the root of a Git repository is because it contains a directory called .git (note the dot in front, which the git directory in Home doesn't have).

If your project files are indeed part of a Git repository, the root would be in one of the upward directories from /opt/lampp/htdocs/d8/web. 


  So is --drupal-root=./opt/lampp/htdocs/d8/web ??


Almost. The drupal root is where the core directory and index.php can be found. But that . in there makes this a relative directory path. You almost certainly mean to use an absolute path there, which would be --drupal-root=/opt/lampp/htdocs/d8/web.


  So is composer-root=./home ??


No. Most likely, you will want your composer-root to be one directory up from your project root (where index.php is). So I'd try --composer-root=/opt/lampp/htdocs/d8.