Hey otopp,
Hope you are having great week so far!
What FTP client do you use to connect to the site?
You will need the correct FTP details in order to connect.
They are different from the ones you are using to log in your WordPress Dashboard.
You will usually find them in your hosting control panel.
Here is a link from GoDaddy KB how to find this - https://support.godaddy.com/help/article/4881/finding-your-hostingftp-login-or-user-name
If you are using different host provider, they should be able to help you with this too.
giveusyourinput.org/wp-admin:21
This is not a valid FTP address.
FTP addresses doesn't include folders, they usaullay look somethi like this:
giveusyourinput.org
or
128.0.0.1
The :21 after them is the port you are connecting with them and may be different for your case.
I will suggest you reading this article - http://www.wikihow.com/Use-FTP
And any other you find interesting on the internet.
Thanks,
Ivan