How to broadcast post from subsite to main site?

It sounds like the user does not have access to the main site?

In that case, you can either add the user to the member list of the main site, or use a code snippet to add blog access or use the All Blogs add-on.

First for add the user to the member list of the main site is success. Thanks .

But for another user for code snippet to add blog access still no result, I put this code in the bottom of functions.php in running theme.

What is the ID of the user, and what is the snippet you used?

function my_threewp_broadcast_get_user_writable_blogs( $action )
{
	$users_and_blogs = [
		// User ID			// Blog IDs
		38				=> [ 1 ]
		//6				=> [ 88, 89, 91 ],
	];
	if ( ! isset( $users_and_blogs[ $action->user_id ] ) )
		return;
	
	foreach( $users_and_blogs[ $action->user_id ] as $blog_id )
		$action->add_access( $blog_id );
}
// Broadcast fills the blog list at prio 100, so use a higher number.
add_action( 'threewp_broadcast_get_user_writable_blogs', 'my_threewp_broadcast_get_user_writable_blogs', 200 );

User ID 38 got from phpmyadmin inside table wp_users.

That looks right to me. I am unable to say what is wrong.

Does adding the user to the user list of site 1 not work?

I can add the user to the user list of mainsite.com ( ID 1).

But this is consume many time if there are thousands users. I must add the users one by one.

How to write code snippet for thousands of User ID to access blog ID 1 ?

Can i write like below ?

function my_threewp_broadcast_get_user_writable_blogs( $action )
{
	$users_and_blogs = [
		// User ID			// Blog IDs
		>=2				=> [ 1 ]
		//6				=> [ 88, 89, 91 ],
	];
	if ( ! isset( $users_and_blogs[ $action->user_id ] ) )
		return;
	
	foreach( $users_and_blogs[ $action->user_id ] as $blog_id )
		$action->add_access( $blog_id );
}
// Broadcast fills the blog list at prio 100, so use a higher number.
add_action( 'threewp_broadcast_get_user_writable_blogs', 'my_threewp_broadcast_get_user_writable_blogs', 200 );

Just replace the contents of the function with one line

$action->add_access( 1 );

That will add all users to site 1.