I am having the same issue. I had restrictions in place prior to IOS12 that included App restrictions. I could turn it on and off with the Restrictions password so my son could not install apps.
I have been using Screen time restrictions since installing IOS12 but now it is not possible to get the App Store back. I have the app store enabled and the age limit also set to unrestricted under Content and Privacy Restrictions. I have done a restart and reset but nothing works. I could wipe the whole phone, but would like to avoid the nuclear option.
It looks like the old Restriction settings are still active somewhere in the code, but the new Screentime settings are not overriding them.
I am on an iphone 6 with the very latest version of ios12.1.1
Apple - this must be a bug!!!!