As Jacob Peattie pointed out in this comment, the reason why you code isn’t working is because you have two functions with same name create_posttype. PHP assumes you want to re-declare the function, which you cannot do.
So in order for your code to work, you can either combine your custom post types creation into one function (as Jacob suggests):
// Our custom post type function
function create_posttype() {
register_post_type( 'articles',
// CPT Options
array(
'labels' => array(
'name' => __( 'Articles' ),
'singular_name' => __( 'Article' )
),
'public' => true,
'has_archive' => true,
'rewrite' => array('slug' => 'articles'),
'show_in_rest' => true,
)
);
register_post_type( 'projects',
// CPT Options
array(
'labels' => array(
'name' => __( 'Projects' ),
'singular_name' => __( 'Project' )
),
'public' => true,
'has_archive' => true,
'rewrite' => array('slug' => 'projects'),
'show_in_rest' => true,
)
);
}
// Hooking up our function to theme setup
add_action( 'init', 'create_posttype' );
… or give each function an unique name if you prefer to use two functions:
// Our custom post type function
function create_posttype_articles() {
register_post_type( 'articles',
// CPT Options
array(
'labels' => array(
'name' => __( 'Articles' ),
'singular_name' => __( 'Article' )
),
'public' => true,
'has_archive' => true,
'rewrite' => array('slug' => 'articles'),
'show_in_rest' => true,
)
);
}
// Hooking up our function to theme setup
add_action( 'init', 'create_posttype_articles' );
// Our custom post type function
function create_posttype_projects() {
register_post_type( 'projects',
// CPT Options
array(
'labels' => array(
'name' => __( 'Projects' ),
'singular_name' => __( 'Project' )
),
'public' => true,
'has_archive' => true,
'rewrite' => array('slug' => 'projects'),
'show_in_rest' => true,
)
);
}
// Hooking up our function to theme setup
add_action( 'init', 'create_posttype_projects' );