This error message indicates the product is supported, the page is in sync and the contents appear to be editable, but no encrypted data was found.
This can happen when you Reply to a message, but the contents are hidden. It can also occur when you try to decrypt a message that doesn’t have any encrypted data in it.
If you are replying to a message, ensure you “show trimmed content” to proceed. This means making sure the message contents are visible in the edit window, not hidden. You’ll also want to be certain the message has encrypted data in it before decrypting it.