I added the CAPTCHA module to my drupal 6 website using this procedure and I'm trying to add the captcha to my user registration page.
However when I go to /admin/user/captcha the list with the default forms is empty. I expected the default list.
I entered user_register as the form_id, selected the standard captcha from the dropdown and pressed save. However it does not add the new form to the list. It's still empty. I also tried to save other values without success.
UPDATE: I also tried reinstalling the captcha module and flushing cache without success. It's weird because I never had this problem on an other site with the same module/version
Installed version of the module: 6.x-2.4
UPDATE 2: I enabled both the Captcha module and the Captcha compatibility module in the modules list.