Price By Country Throwing Fatal Error

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function “save_my_custom_field” not found or invalid function name in /home/nomorehangmy/public_html/wp-includes/class-wp-hook.php:326 Stack trace: #0 /home/nomorehangmy/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #1 /home/nomorehangmy/public_html/wp-includes/plugin.php(517): WP_Hook->do_action() #2 /home/nomorehangmy/public_html/wp-content/plugins/woocommerce/includes/admin/class-wc-admin-meta-boxes.php(273): do_action() #3 /home/nomorehangmy/public_html/wp-includes/class-wp-hook.php(326): WC_Admin_Meta_Boxes->save_meta_boxes() #4 /home/nomorehangmy/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #5 /home/nomorehangmy/public_html/wp-includes/plugin.php(517): WP_Hook->do_action() #6 /home/nomorehangmy/public_html/wp-includes/post.php(4891): do_action() #7 /home/nomorehangmy/public_html/wp-includes/post.php(4993): wp_insert_post() #8 /home/nomorehangmy/public_html/wp-admin/includes/post.php(453): wp_update_post() #9 /home/nomorehangmy/public_html/wp-admin/post.php(227): edit_post() #10 {main} thrown in /home/nomorehangmy/public_html/wp-includes/class-wp-hook.php on line 326

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I am receiving this error while updating product price as a fixed rate.