In your code where you defined your custom post type (in a custom plugin or in your functions.php
file), you need to add this snippet for Gutenberg block editor support:
'show_in_rest' => true,
'supports' => array('editor')
Here is an example:
function portfolio_post_type() {
register_post_type( 'portfolio',
array(
'labels' => array(
'name' => __( 'Portfolio' ),
'singular_name' => __( 'Portfolio' )
),
'has_archive' => true,
'public' => true,
'rewrite' => array('slug' => 'portfolio'),
'show_in_rest' => true,
'supports' => array('editor')
)
);
}
add_action( 'init', 'portfolio_post_type' );