This error might mean that admin$ (admin share) is not allowed on the remote machine or you don't have access to it. In addition, this error might also be caused by the fact that the disc Z is mapped on the Viewer computer and, therefore, is not available. Please make sure that the Z drive/letter on your Viewer PC is empty and available because the program uses that drive temporarily for the installation/upgrade process.
Could you please clarify what Connection type do you use when connecting to the remote machine via the Remote Install Tool? In case if you use Remote Utilities security, please try using the Windows NT security instead, as it allows you to connect to a remote computer using admin$ share.
In addition, please note that since you've mentioned above that you're connecting to Agent running on a remote machine, this might also be causing the issue as Agent runs as an application under the current Windows user without administrative privileges. Please try restarting Agent as administrator first using the Restart Agent as command and using the Remote Install Tool via Remote Utilities security (in case if you're connecting to the remote machine over the Internet) once again.