Hi Patrick,
Thanks for getting in touch, and for searching the forums before posting your own thread. We always appreciate that!
You can choose to use an Unlimited license on a multi-site install if you are willing to handle upgrades and licenses on a sub-site level. The Unlimited license will not work on the network level or allow network-level activation. Generally folks with a multi-site want to be able to take advantage of network management features, and thus they would have to purchase a multisite license.
The notice in your screenshot above is only meant as an alert- it won’t prevent you from using any features or updating the site.
Does that help clarify things? If you would like to switch to a multisite license instead of the unlimited one, let us know and we’d be happy to help make that happen.
Best,
Leah