I am converting a normal site, already set up in Local, to a multisite. Setting up the multisite is fine, but Local still says it’s not a multisite, so domains cannot be synced and any new subsites added cannot be used. Restarting the site and the app make no difference.
Does this happen for all sites in Local, or just one in particular? Every site that has not been set up as a multisite initially that you then convert into a multisite, yes.
Are you able to create a new, plain WordPress site in Local and access it in a Browser? Yes, but irrelevant
Right now, Local can’t switch between the two types of sites (regular/multisite). Instead, you’ll need to create a new site in Local and mark it as a Multisite, and then import the multisite into this new site. This reply and topic has more info about the details: