Hello,
Is this issue occurying only on 1 of the machines?
Could you attach the complete SEP_Inst.log file?
I believe you are trying to install SEP client by deploying the client on the machine... what happens if you run the setup.exe package on the machine locally?
A quick check on this Article:
Symantec Endpoint Protection 12.1 client install rolls back, returning 1603
http://www.symantec.com/docs/TECH170259
Awaiting your reply.