You have to download and install FortiClient from the Microsoft Store. It's an outdated version and appears to only support VPN functionality but it seems like the only option for ARM processors at this time.
I had that issue with a Microsoft Surface X. I tried installing:
- various versions
- restarting the pc in-between attempts
- running in different compatibility modes
- running as administrator
- disabling User Account Control
- disabling firewalls and antivirus protections
- installing Microsoft .NET Framework 3.5 and 4.5.2
None them worked... when trying to install the drivers... it fails and rolled back every time. I installed the version from the Microsoft Store and that solved it.