We don’t allow multisite on our Starter plan because these types of sites are generally more resource-intensive and require additional support. All subsites in a multisite network share the same WordPress database and installation, which means one install could consist of dozens of sites.
As setting up multisite correctly is complicated and requires special attention, you should anticipate working with a qualified WordPress developer if you wish to switch an established site from single-site mode to multisite mode.
If you opt for a subdirectory multisite after creating a subdomain multisite, a rule will need to be added by the Kinsta support team for it to work properly.
Make sure to also check out our post on an introduction to WordPress multisite, and learn about some of the pros and cons of this setup. In some instances, you might be better off managing them with a third-party tool and keeping the sites completely isolated. You can also learn more about how multisite works by reading our WordPress Staging Environment Guide.