This error appears when you have at least one
label element in your form that invalidly contains two or more form controls. A label must be corresponded to a single form control.
This is often caused by wrapping checkboxes or radio buttons with
<label></label>, such as in the example below:
<label>[checkbox your-country "China" "India" "San Marino"]</label>
Contact Form 7 provides a
use_label_element option for labeling checkboxes and radio buttons. Use this instead of
Contact Form 7 4.5’s release announcement describes the story behind the introduction of this validation.