phpdocx is fully tested with PHP CLI mode and almost all web servers (IIS, apache, lighttpd, , nginx, Cherokee, ...), OS (Linux, Windows, macOS) and PHP handlers (mod_php, FastCGI, php-fpm, CGI...). So some rw access should be missing (IIS may need setting a custom temp_path with rw access in the config/phpdocxconfig.ini file).
As first test, after you have added the license key and copied all files, please run the most simple sample included in the package (Core/addText/sample_1.php) using the PHP CLI mode:
C:\...> php.exe sample_1.php
Please note that you may need to set the absolute path to php.exe if it's not as an env variable. The previous script must generate a DOCX in the same folder than the script; otherwise please check your server logs (maybe some PHP extension is missing for PHP CLI mode).
When you run it using IIS, what error do you see in the server logs? As an Internal Server Error is a generic error, the server logs should explain what's happening.