This status message tells you this is a supported product, but there isn’t any message content able to be encrypted or decrypted yet. (This can happen when you haven’t Composed a new message, or haven’t Forwarded an existing message, etc.)
When PrismCipher encrypts or decrypts, it actually overwrites the existing message, but it can’t do that unless the contents can be changed.
The easiest way to use PrismCipher is to Compose a new message and then encrypt it. You could also Forward an existing message and encrypt it before you hit send. Both of these options create editable content.
Optionally, you can Reply to a message, but you must “show trimmed content” by making the message visible in the edit window. If you can’t see the message, PrismCipher won’t be able to edit it.