Why is Microsoft Windows Defender service not starting?
SEP will keep the Windows Defender service from starting due to compatability issues between the two applications.
Running the Windows Defender service on a computer with SEP installed is unsupported. This is because of compatability issues between the applications.
At this time, you cannot run both Symantec Endpoint Protection and Microsoft Windows Defender as it mistakes Symantec Endpoint Protection for malware. For this reason, Microsoft Windows Defender is disabled when Symantec Endpoint Protection is installed.