In a Spring Boot project with Java 1.8 I’m having difficulty in correct communication between the controller which displays the values ​​correctly and the front-end part which doesn’t seem to be able to receive them correctly.

So i’m new to Spring Boot and i’ve been reading the 5th Edition Spring in Action book, by Craig Walls. In Chapter 2, section 2.3, it says to add a couple lines of code to validate the information input on the design taco form and the order form so it would show the user what is wrong in the form. I did exactly what was on the book but it shows a WhiteLabel error page instead.