Failed to Connect to FTP Server

I continue to get this error:
Failed to Connect to FTP Server mydomain.com :21 whenever I try to change the theme or add a plugin. I browsed the forums to try and figure out how to fix it and frequently saw this article listed as a solution:
https://www.nerdgrind.com/wordpress-automatic-upgrade-plugin-failed-or-not-working/

However, I can’t figure out what it is telling me to do or it is not working. If someone could explain to me in simpler terms how to fix this problem it would be greatly appreciated!