Dynamic display of suggestions in the search bar with Spring Boot [closed]

I have a spring boot project that deals with the management of books and authors, in particular in the form for entering a new book I must also add its author or several authors associated with that book.

To enter an author I use a search bar which checks based on the first letters typed in the bar which author or authors in the db have those characters entered.

So it must be possible to display suggestions when typing letters into the search bar.

I have implemented the search bar, the problem is that no hints are displayed.

What happens instead when I start typing letters is that I get an error message instead of the suggestions.

Here are my codes I wrote:

https://gist.github.com/reginelsunshine/1859953f616275652e36d394ee4698fa#file-gistfile1-txt