The instructions for FTP are detailed in this FAQ (there is more than one method, but what I used is the simplest): https://galaxyproject.org/ftp-upload/
To troubleshoot, read the FAQ help and double check the settings against the instructions - server name, email address, and password must all be correct. Be aware that Galaxy accounts/credentials (email, password) are server-specific. Most people who have trouble are not using passive FTP or have used an incorrect server name and/or account creds.
For Galaxy Main, the server name is simply
usegalaxy.org (no prefix). For other public Galaxy servers, the base URL is also the server name (no prefix). Not all public Galaxy servers will have FTP enabled. If you are not sure if the target server has FTP enabled, contacting the admins of the server you are working on is a good way to find out if FTP is active and working.
If FTP continues to fail, there may be a firewall where you are working preventing the connection. Check with your local admin or ISP provider for advice (how to get around a firewall can vary).
All support FAQs: https://galaxyproject.org/support/
Hope that helps! Jen, Galaxy team
If you're still unable to upload after using the resources Jen posted, please provide specific details. Most helpful would be the settings you are connecting with - active vs. passive, implicit SSL vs. explicit, ports, etc., and any error messages you are receiving, where in the connection process you receive them. Screenshots are useful if you are using a graphical client.