Settings Errors

<?php

add_action( 'add_meta_boxes', 'acme_add_metaboxes' );
function acme_add_metaboxes() {
    add_meta_box( 'acme-metabox-mandatory-field', __( 'Mandatory Field', 'acme' ), 'metabox_mandatory_field', 'acme', 'normal', 'high' );
}

function metabox_mandatory_field( $post ) {
    settings_error( 'mandatory-field' );

    echo '<input type="text" name="mandatory-field" value="' . get_post_meta( $post->ID, 'mandatory_field', true ) . '">';
}

add_action( 'save_post', 'acme_save' );
function acme_save( $post ) {

    // Classic veririfications here

    // Check for mandatory field
    if ( isset( $_POST['mandatory-field'] ) ) {
        if ( ! empty( $_POST['mandatory-field'] ) ) {
            update_post_meta( $post->ID, 'mandatory_field', sanitize_text_field( $_POST['mandatory-field'] ) );
        } else {
            add_settings_error( 'mandatory-field', '', __( 'You must define a value in the metabox.', 'acme' ), 'error' );
        }
    }
}