custom metabox data is not saving

<?php

/**
 * Adds a box to the main column on the Post and Page edit screens.
 */
function offer_add_meta_box() {

    $screen = 'offers';

        add_meta_box(
            'offer_meta',//section id
            __( 'Offer Details' ),//section heading
            'offer_meta_box_callback',// call callback function
            $screen//post or page to show
        );//end of add meta box function
}
add_action( 'add_meta_boxes', 'offer_add_meta_box' );//adding to action meta box

/**
 * Prints the box content.
 * 
 * @param WP_Post $post The object for the current post/page.
 */
function offer_meta_box_callback( $post ) {

    // Add an nonce field so we can check for it later.
    wp_nonce_field( 'offer_meta_box', 'offer_meta_box_nonce' );

    /*
     * Use get_post_meta() to retrieve an existing value
     * from the database and use the value for the form.
     */
    $value = get_post_meta( $post->ID, '_my_meta_value_key', true );

    echo '<label for="offer_details">';
    _e( 'Offer Details' );
    echo '</label> ';
    echo '<textarea id="offer_details" name="offer_details" value="' . esc_attr( $value ) . '" size="25"></textarea>';
}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function offer_save_meta_box_data( $post_id ) {

    /*
     * We need to verify this came from our screen and with proper authorization,
     * because the save_post action can be triggered at other times.
     */

    // Check if our nonce is set.
    if ( ! isset( $_POST['offer_meta_box_nonce'] ) ) {
        return;
    }

    // Verify that the nonce is valid.
    if ( ! wp_verify_nonce( $_POST['offer_meta_box_nonce'], '_meta_box' ) ) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Check the user's permissions.
    if ( isset( $_POST['post_type'] ) && 'offers' == $_POST['post_type'] ) {

        if ( ! current_user_can( 'edit_post', $post_id ) ) {
            return;
        } 
    }

    /* OK, it's safe for us to save the data now. */

    // Make sure that it is set.
    if ( ! isset( $_POST['offer_details'] ) ) {
        return;
    }

    // Sanitize user input.
    $my_data = sanitize_text_field( $_POST['offer_details'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, '_my_meta_value_key', $my_data );
}
add_action( 'save_post', 'offer_save_meta_box_data' );