call support and get cleanwipe tool
run the cleanwipe and then install SEP
or
Cleanwipe is avaialble in file connect.
You can go with valid key
https://symantec.flexnetoperations.com/control/symc/registeranonymouslicensetoken
Download the tool part2.exe from file connect.
Extract it and copy the cleanwipe from there.
or
download norton removal tool and run it before installign sep
https://support.norton.com/sp/en/us/home/current/solutions/kb20080710133834EN_EndUserProfile_en_us