how to error_log(wp_login_url())

I think, while saving the post, you won’t be able to display this result.

You can try below instead –

Log the output in the database.
e.g. update_option('some_option', wp_login_url($redirect));

And then print get_option('some_option') directly somewhere outside this save_post hook callback (e.g. on wp_footer hook). You may need to refresh the page after save post to see the result.

or rather just check directly in the options table of WordPress database to see the result.