I just created a test ftp account under your domain and it works fine when I test it, so the server FTP system is working with no problems.
Let's break this down and hopefully make it easier for you to follow:
1) The USER NAME that you should be using is identical to the user name for your cPanel. This is not something that you set. You can find your cPanel user name here: How find your account technical information
2)The user name in that article is set to "inmoti6". Your user name will resemble this format. This should be the SAME user name and password used to login to the cPanel
. Make sure your user name is set correctly.
3)Additionally, you indicated that you put 'public_html' in the directory section, but the instructions state that entry should be blank. Please change this as well.
I hope this does the trick! I was able to confirm that FTP is definitely working on your account, so hopefully you will be able to confirm it on your end. Let us know if you need any further assistance!