Let's Try Let's Encrypt

If you get this

gpg: keyring `/tmp/tmptwv64yji/secring.gpg' created
gpg: keyring `/tmp/tmptwv64yji/pubring.gpg' created
gpg: requesting key 75BCA694 from hkp server keyserver.ubuntu.com
gpgkeys: key YOURKEY can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: keyserver helper general error
gpg: keyserver communications error: unknown pubkey algorithm
gpg: keyserver receive failed: unknown pubkey algorithm
Failed to add key.

Try this

sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com YOURKEY