After the searching, I found "421 Sorry, cleartext sessions are not accepted on this server" means that the server is using "FTP over Explicit TLS/SSL".
Filezilla might be a good choice for connecting "FTP over Explicit TLS/SSL"
Here are the steps to connect FTP over the TLS
- Download Filezilla using the URL https://filezilla-project.org/
- Install FileZila into your local machine
- Click on File Menu >> Site manager
- Click on the New Site Button
- Enter the Hostname, port, Username and Password
- Select the encryption option "Required explicit FTP Over TLS"
- Click on Ok Button