File upload failing

I'm trying to upload a signature to Files and I don't understand why I'm getting a "List has no rows for assignment to sObject" error. What row am I assigning to what object?

I am trying to upload a users signature from this Experience Builder form. The Experience Builder form uses a flow called Candidate Application, which is the flow that contains this Signature component. The Signature component is an Aura component, and the Experience Builder page is an Aura framework, so I don't think there is a problem there. And when I run the flow directly (as the administrator of the form and not as a guest user), the signature uploads fine. It just doesn't upload when I run the flow as a guest user. So I don't understand why.

https://forcepanda.wordpress.com/2019/12/10/signature-capture-component-for-flows/

This is the package I downloaded to get the Signature component I'm talking about

#Upload Files