Hello, @bold-promo,
In the Porto theme, you can enable maintenance mode for your site. To find this option, navigate to Porto >> Theme Options >> General. For reference, check this screenshot: https://postimg.cc/B898RGcC. Activating this option will set the site to maintenance mode, preventing users from accessing or previewing it.
If you need password protection instead, consider using a third-party plugin from WP Market. Here are a couple of plugins you might find useful: https://wordpress.org/plugins/password-protected/ and https://wordpress.org/plugins/password-protect-page/
Please use these at your own risk.
Best Regards,
PThemes Team.