Problem - someone has complete a Forms submission, but they haven't gotten a confirmation email.
The best first thing to check is going to be any and all spam or junk folders in their email, and do a global search in their email for the sending email address.
Chances are pretty good that this will find the email.
If this is the case, we'd recommend you review Outbound Email Settings in Soapbox and Best Practices to Ensure Deliverability for tips on increasing email deliverability.
If not, please open a support ticket and we will help you look into it further.