RSS: A feed could not be found at `https://planet.wp-persian.com/feed/`; the

See if this tutorial points in the right direction: https://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-rss-feed-errors/

Your site currently has this error message:

“Your server is running PHP version 5.6.40 but WordPress 6.5-alpha-57175 requires at least 7.0.0.”

You need to address that and have a functioning WordPress site before worrying about the RSS feed.

I read this article and it was not helpful to solve my problem

My php version is currently 7.4.1 while this error states that my version is 5.6.40

Create a PHP file on this subdomain, and put:

<?php

phpinfo();

Open this link in your browser to confirm the version.

Go to your hosting dashboard, and if they offer “Multi PHP versions”, change the PHP version for this subdomain.