add_action( 'template_redirect', 'check_if_loggedin' );
function check_if_loggedin()
{
if(!is_user_logged_in())
{
echo '
<script>
alert("You must be logged in to access this page.
You will be redirected to the login page.");
window.location.href = "https://example.com/login";
</script>
';
}
}