Windows 10 has been out for some time now. Upon first being released, Symantec Endpoint Protection clients that were on Version 12.1.5 or older were no longer compatible if upgrading to Windows 10.
Symantec released SEP Version 12.1.6 for Windows 10 users. Fast forward to today and Microsoft has released a Feature Update to Windows 10, version 1607. In order to install the update, users running SEP 12.1.6 are prompted to uninstall SEP because it is no longer compatible with the newer version of Windows 10.
There is however a new version of SEP client 12.1.6 that is compatible with Windows 10. Symantec Endpoint Protection 12.1.6 (12.1RU6 MP6) build 7061 (12.1.7061.6600) for enterprise users can be downloaded here by entering your serial number and downloading the file named Symantec_Endpoint_Protection_12.1.6_MP6_All_Clients_EN.zip.