Hi,
This problem occurs when we used password protected SEP that's why when you run the smc -stop command the SEP asked for the password.
If you don't know the symantec service stop password.
The simple solution for this type of problem is you
You will be able to turn this off in SEPM by going to Clients > Policies tab > General Settings > Security Settings tab > "Require a password to stop the client service". Then Unmark the all password protection and click ok and assign the policy to all group.
Restart the machine on which the SEP is installed and then again try to stop the Symantec services by command.
your problem will be resolved.
Baljeet Singh