Gravatar Image is not showing in Custom Author Box

I have created an Author box using these codes.

Added a Hook at after_content:

<div class="author-box">
	<?php 
	
	global $post;
	
	$author_id = $post->post_author;
	
	?>
	
	<div class="avatar"><?php echo get_avatar( get_the_author_meta( $author_id )); ?></div>
 
        <h5 class="author-title"><?php printf( esc_attr__( 'About %s', 'the author' ), get_the_author_meta( 'display_name') );?></h5>
	
				<div class="author-summary">
	
        <p class="author-description"><?php echo wp_kses( get_the_author_meta( 'description' ), null ); ?></p>
 
        <div class="author-links">
 
            <a>" title="Read more">
                ...
            </a>
 
        </div>
 
    </div>
 
</div>

and this CSS:

.author-links a {
    font-size: 3em;
    line-height: 0.5em;
    float: right;
}

.author-box {
    padding: 4%;
    margin-top: 60px;
    font-size: 0.8em;
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -ms-flex-wrap: wrap;
        flex-wrap: wrap;
}

.author-box .avatar {
    width: 30px;
    border-radius: 100%;
    margin-right: 20px;
}

h5.author-title {
    margin-bottom: 0.5em;
}

Everything is working fine except for Gravatar images of the author.

Please help me with this.