Hello,
What version of SEP 12.1 are you installing? Is that SEP 12.1 RTM or SEP 12.1 RU1?
Could you try visiting the Device Manager > View > Show hidden devices > Check if you have any yellow exclamation OR Red X under "Network Adapters" Or "Non-Plug and Play Drivers"?
Also try removing the Network Threat Protection (NTP) component, then reboot the machine and install it again.
- Windows Control Panel > Add or Remove Programs > Symantec Endpoint Protection > Change > Modify > Network Threat Protection > Entire feature will be unavailable.
- Reboot
- Windows Control Panel > Add or Remove Programs > Symantec Endpoint Protection > Change > Modify > Network Threat Protection > Entire feature will be installed on local hard drive.
- Reboot
OR
TRY installing the SEP 12.1 RU1 and check if that resolves the issue.
If the issue still persists, Try Running the Cleanwipe version 12.1 (available with Technical Support)
https://www-secure.symantec.com/connect/forums/how-get-cleanwipe-tool
Hope that helps!!