10.17. I get an error whenever I try to post to a newsgroup

You are trying to post an encrypted message to a newsgroup. This doesn't make any sense as a newsgroup, like a mailing list, is a public space and not an entity that could own a key pair. (Just ask yourself: who is supposed to own the private key? What would be the trust associated with this entity?)
Send your message unencrypted. If you just want to obfuscate information, like spoilers, you can just as well use ROT13.