You will only be able to sign messages with imported addresses and with addresses created in old Blockchain.info wallet. If you created a new wallet/account in Blockchain.info, you *won't* be able to sign messages with the addresses created.
Okay thanks for clarifying.Yes I'm using an address made in the old blockchain.info wallet before they updated.Never knew newly created addresses don't have signing provision.
Blockchain.info now uses hierarchical deterministic(HD) wallets. Many HD wallets don't allow you to sign messages. You will have to use tools like
https://github.com/iancoleman/bip39 to first find private key of the address you want to sign message with and then import address to a wallet or use a tool.