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First of all, sorry for the inconvenience. I've tried the same scenario on my side, but I didn't replicate the same error.
Also, I found that you have no "E-mail" field in your form. Without it, the reply button won't work on "Submissions" page. Please, add one to your form and try again.
While this problem is blocking one way to reply user's submission, you can do this in a different way.
Alternatively, you can add user's email who filled the form to your incoming submissions. With that, Upcoming email submissions come with the users' email. So, you can reply those emails directly from your inbox.
If you want to reply incoming submission email from your email box, you need to set your "Reply-to Email" setting as a email field's name which is in your form.
To do that,
1) Open the current "Notification".
2) On the appeared window, click to "Reply-To and Recipient Settings".
3) On that screen, change the "Reply-to Email" as a email field's name which is in your form
I hope this one will help you. Please give us feedback about the issue.