Why is “una” required?

Hola Shirley

The way we use the word for "news" is a bit different to how it's used in English. In Spanish we also use it in singular (una noticia, dos noticias...) while in English the singular is not used. This is why we use here the article: ...es una buena noticia. (That Lisa is pregnant is a good piece of news)

You may also see something like: "Eso son buenas noticias" where we have the singular "Eso" and the verb and news in plural. And it's common to say for example: "Traigo buenas noticias" when you want to say "I bring good news", and what you bring is just one piece of good news, but in this case it's used in plural (although you can also say "traigo una buena noticia". 

So, the important thing here is to use the article if you decide to use "news" in singular in Spanish.

Saludos

Inma