There seems to be an intermittent product issue with the controls used in user views. What that error message says is the system is having difficulty rendering the pxTextInput control associated with the property .FirstName when the Authenticate Existing Customer flow reaches the assignment (collect information step) labelled internally as Assignment1. (It doesn't help matters that you need to parse an ugly, unformatted error string to try and identify the root cause of the issue.)
You should be able to work around the issue by opening the section being used, and configuring the field to revert to the control defined on the property rule, rather than explicitly setting the field to use the Text Input control. (The property uses the same control - Text Input - if you configured the field to store text values.)
If you reply with a link to your exercise environment (either on this forum post or in a private message), I can investigate a little more and confirm the previous assessment of the issue.
Which version of Pega Platform are you using? We've observed this behavior in version 8.2, but I don't recall hearing of a case occurring on version 8.1, on which the SAE course was released.