Here's why:
* "News" is an uncountable noun. This means it doesn't have a plural form. You wouldn't say "two news," you'd say "two pieces of news."
* "Good" is an adjective, and adjectives modify nouns. You can't directly say "good news" because "good" doesn't have a plural form to match "news."
Here's how to correct it:
* "This is good news." (If you're referring to one piece of news)
* "These are good pieces of news." (If you're referring to multiple pieces of news)
* "This is great news." (Alternatively, you can use a different adjective like "great" or "wonderful.")