When the computer is not in a Domain and you must reach the admin$ share with a local administrator you have to edit the registry.
Test if you can access ADMIN$ share with a user who is member of the local administrators group.
If not, test it with the standard built-in administrator account. If these works try the following registry setting:
You need to add a new DWORD value in the registry.
Run -> Regedit
Navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
Add a new DWORD Value called: LocalAccountTokenFilterPolicy and give it the value of 1.
Reboot the system.