Installing software typically requires elevated permissions that standard user accounts do not have. The CompTIA ITF+ FC0-U71 Official Study Guide explains:
"An administrator account provides full system access, including installing or removing software, changing system settings, and managing user accounts." The user account does not have these privileges by default.
Least privilege refers to granting only the minimum permissions necessary (which would not typically include software installation for regular users).
Single factor refers to authentication methods, not access levels.
Since the employee is denied installation due to insufficient privileges, administrator access is needed to complete the installation.