Did you try adding {{ formData }} into the email template in the project settings like @theecarls recommended?
Since you’re saying that the users can only enter their email into the form, the confirmation-email you receive should look like this:
You just got a form submission!
Form
Email Form
Site
my-portfolio-website
Submitted content
email: john.doe@xy.com
Number of submissions received
1/50 this month
January 1st – January 31st
Need more submissions?
Please contact your website administrator.
Unsubscribe from notifications for this site.
Does the email you receive look something like that?