Captcha is a security feature mostly used for a login screen. Are you trying to add this to Pega Login screen? If so, then you may just go to Syetem->Settings->Security Policies. There you enable security policy and you may add a captcha from there. But here you are trying to add a captcha in a section. First thing is why that is required? If required then you might try to use a non auto-generated section. There you might try to add 2 HTML fragments named pyCustomCaptchaHead and  pyCustomCaptchaBody. To include them you might add the below code 

<pega:include name="pyCustomCaptchaHead" type="Rule-HTML-Fragment"/>

<pega:include name="pyCustomCaptchaBody" type="Rule-HTML-Fragment"/>

Hi Venkata,

Please find the below article which will help you:
https://docs-previous.pega.com/security/85/customizing-captcha-presentation-and-function

Regards,
Evita.