How to get the user meta data for a post?

Question

You can pass the post’s author as an argument to get_the_author_meta:

get_the_author_meta('ID', $post->post_author);

The second argument is the user’s ID. This is stored in the post object in your loop, which you can access it by using $post->post_author.

Reason

The reason behind the current code of yours that isn’t working is this piece of code that is included in the get_the_author_meta():

if ( ! $user_id ) {
    global $authordata;
    $user_id = isset( $authordata->ID ) ? $authordata->ID : 0;
} else {
    $authordata = get_userdata( $user_id );
}

If you set the second argument to true ( which is the $user_id ), it will trigger the else, and by triggering the else you are passing a true to the get_userdata(), which will obviously won’t work.

Take a look at this page of code reference for more details.