Littlegolden The extension isn't on Packagist, so you'll have to make sure the repository is present in a folder called post-license inside workbence. Then you'll have to run:
composer require clarkwinkelmann/flarum-ext-post-license *@dev
You will also have to install node_modules to the post-license/js/ directory.
Edit: After writing this I realized you have already done all of that.
What happens when you run this?
composer require clarkwinkelmann/flarum-ext-post-license *@dev
Symlink failed could mean that public/vendor/clarkwinkelmann/flarum-ext-post-license does not have permission to access workbench/post-license but I'm not sure, never faced this problem. Also, check composer.json to see if there are any errors (I doubt there'll be any, but still).
