This article covers the captcha which is displayed when you choose the option "Visforms Captcha" in the form settings.
The so called Visforms Captcha uses the free open-source PHP CAPTCHA Script. Characters, displayed in the captcha image, are created through php code by the script. To achieve this, the script has some php system requirements which sadly are missing in some rare cases. Especially the PHP FreeType support, is such a setting, that must be installed and enabled.
If the FreeType support is missing, the captcha image cannot be created and therefore it cannot be displayed either. If you have enable the "Debug System" option on your Joomla! installation you may get the following error message: "Call to undefined function imagettftext()" in securimage.php.
You can check in the administration of your website, if the FreeType support is installed and enabled. Go to System menu and then to System Information. Click the PHP Information tab. Use the browsers search function and search for FreeType to check if FreeType support is installed and enabled. You can open the browser search function by pressing the "Ctrl" + "f" keys.
If FreeType support is not installed or not enabled please contact your provider in order to ask if it is possible to enable it. If not, you cannot use the Visforms Captcha. Check out Google Recaptcha instead or start using the very effective Plugin Visforms Spambotcheck which is part of the Visforms installation.
On the PHP CAPTCHA Script authors website there is a FAQ listing known problems and solutions which might be helpful as well.