That error should go away if you
1) Ensure that you have correct web address set up for your account profile (see Tools->Upload/Manage). A correct web address typically looks like this:
https://www.yoursitehere.com
2) Connect to your account using Tools->Upload/Manage, then locate the folder that represents the root of your web site (typically called public_html, http_docs, www, yourdomain.com etc)
3) Right-click that folder and select "Mark as web root"
Now you should be able to upload without that "Unreachable at ..." error