Contact Form Required Fields Plugin – formEntry error
The plugin expects the the ID of the entry you are using to build the form fields to be submitted along with the form so it is available as a request variable. 

You should include the ID of the entry you are using to build the form’s fields when you are submitting to the contactForm/sendMessage controller. For example:

{% set form = craft.entries({
    section: 'forms',
    slug: 'contact-form',
}).first() %}

&lt;form method="post" accept-charset="UTF-8"&gt;
    &lt;input type="hidden" name="action" value="contactForm/sendMessage"&gt;
    &lt;input type="hidden" name="formId" value="{{ form.id|hash }}" /&gt;
    ...
    {# Loop through the form fields here #}
    &lt;button type="submit"&gt;Send&lt;/button&gt;
&lt;/form&gt;


The code above assumes you have a section with the handle forms and the slug of the entry you want is contact-form.

If you want a more robust solution that provides the form builder and validation out of the box, you might consider checking out Freeform from Solspace.