The Tor project maintains its own directory service of relay nodes, which users of the Tor network download. From there, the client software uses various algorithms to select which nodes listed in the directory to use for connections.
This part of the error message:
Your cache administrator is webmaster.
Suggests that whatever you're pasting the onion address into is configured to use a caching proxy that does not understand onion addresses. You'll need to reconfigure your system to use a proxy that understands onion addresses, or to not use a proxy for connecting to onion addresses....
Onion services come and go quickly. It is no surprise that many of the ones that you might use are not functional. If you want to test your connection, a couple of ones that you can try are: facebook and the tor project.