If you have a PC/Laptop then please download the electrum wallet or bitcoin core from the original site. You will sign message with that wallet on your Laptop because if you are using android wallet then you may not be able to do it "or I missed something". Or if you have Electrum on Android, then you can still export seed from your wallet to Electrum on your Laptop without having to create a new wallet and start signing message. Good luck, good luck.

Follow the steps here: https://bitcointalk.org/index.php?topic=990345.0