Getting a GET error in console – 404 not found for wp-json

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The URL you’re using is not the URL that you’ve registered.

This code:

register_rest_route( 'news', '/news', array(
    'methods' => 'GET',
    'callback' => 'custom_api_get_news_callback'
));

Registers this URL:

http://localhost:8888/una/wp-json/news/news

But you’re sending requests to

http://localhost:8888/una/wp-json/news/all-posts

Nowhere in your code have you defined an all-posts endpoint.

You either need to change /news in register_rest_route() to /all-posts, or the /all-posts in your URL to /news. They need to be the same.