CAN I lock sys or system user in oracle

I've ECC 6.0 system with Oracle 12C
My security auditor keeps saying I should lock sys and system account in DB.
I know I can still log on to SYS user by as sysdba.

I've locked those 2 users in dev system and it does not seem to have any problem.

Can I lock sys and system user in oracle?. Will it cause any problems?

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