OTP generated, but unable to send mail.

I wanted a functionlaity of otp verification during registering to the website, so I am using User verification's OTP login feature. I displayed form in register page. On clicking send OTP button in form an error comes "OTP generated, but unable to send mail." Please help in resolving this error.

I have also a query that I want more fields with this form so how do I add this shortcode/form?