I got this message in April; my assumption is that the bug is not yet fixed
This is a known bug, that the incorrect message will appear when you attempt to subscribe, but are already subscribed.
It will be corrected in a future release of Jetpack.
Thanks for bringing it to our attention. If you’d like to follow along with the bug report, you can see it on GitHub where we track Jetpack’s issues and improvements:
https://github.com/Automattic/jetpack/issues/3110
Thanks,
Lisa