Hey,
yeah, well, if you want to use Let’s Encrypt, you’ll have to make your local web server reachable from the Let’s Encrypt servers (meaning from the internet). Until you decide to really use it, you should indeed remove the app again. You can do that from the command line, too: univention-app remove letsencrypt
After the removal the file /etc/apache2/sites-available/univention-letsencrypt.conf and the symlink in …/sites-enabled/… might still exist; in that case you should remove them manually. Then start Apache again.
m.