Apple introduced a new form of protection for system locations (SIP) in this release that can prevent the Popup Client from installing. This is due to a supporting file called pharos.convs that can only be written to a system location at this time. Apple has added an exception to allow the installer to bypass SIP for this particular file. The exception is not part of the base OS install, but is retrieved from an Apple network service. Machines that have automatic updates disabled may also not update the exception list.
Most machines operated by individuals should have had a chance to retrieve the list of SIP exceptions before an attempt to install the Popup Client. For machines being imaged automatically, the following commands can be used to force an update of the exception list:
sudo softwareupdate --schedule on
sudo softwareupdate --background-critical
The first command enables automatic background updates, if necessary. The second forces an update of critical system protection data, including the exclusion list for SIP. Note that this is currently an undocumented parameter for the softwareupdate tool. The best way to configure a single system is to make sure both the 'Automatically check for updates' and 'Install system data files and security updates' boxes are checked in the App Store pane of the System Preferences application.