Following guide http://docs.electrum.org/en/latest/coldstorage.html I intended to spend bitcoins from paper wallet signing transaction on offline machine. So I created Electrum wallet using given private key. However button Wallet -> Master Public Keys is grey, so I can't obtain PubKey and create a watching-only version on online machine. What is the case?

What kind of Public Key is actually needed? I obtained uncompressed -130 characters and compresed -66 characters from bitaddress.org (offline) but it won't go through.

Electrum-2.9.2
Ubuntu-16.04.2