I am working for one of my client. Basically the task is for subscription form like this
https://lumity.com/blog I am unable to pass any email. What should be step by step guide in Marketo for subscription form?
A form "like" that form? Or that form?
That form isn't a Marketo form. It's a custom HTML form that posts via Ajax to the endpoint https://lumity.com/wp-json/contact-form-7/v1/contact-forms/4028/feedback . That endpoint is broken and doesn't accept even valid email addresses, which is why the form displays an error on any submission. Nothing to do with Marketo though.
So show your actual form. How could we troubleshoot your form if you're showing us an entirely different page + form, which also happens to be broken... doesn't make sense!