You're getting this error because the username or password on the SFTP tab of the Sync Tool is incorrect.
Double check the information entered into the tool is matching what was given to you by your implementation consultant or by the CS team.
It's possible the information you have on file is out of date and the username or password was updated since. Reach out to support if you'd like the information again.
Search for blank spaces in the Username field. Sometimes people copy and paste with extra space characters in error into either the Password or Username field.