Oracle: unable to change to noarchivelog

We discovered that our Oracle 12c database was taking up a large amount of disk space with archive logs, so I figured out how to delete the files, but I'm unable to disable archive logging. We don't have an in-house DBA, so I'm trying to figure this out myself. From lots of Googling, it's my understanding that I need to do the following:

sqlplus / as sysdba
shutdown immediate
startup mount
alter database noarchivelog;
alter database open;

I've got two problems though. I can't do the "sqlplus / as sysdba", it complains about "ORA-12560: TNS:protocol adapter error" unless I specify the SID, even though it the environment variable is defined. Also, when I do the shutdown, I get "ORA-12514: TNS:listener does not currently know of service requested in connect descriptor" and it disconnects me from Oracle.

C:\Users\administrator.MYDOMAIN>set oracle_sid=orcm

C:\Users\administrator.MYDOMAIN>sqlplus /nolog

SQL*Plus: Release 12.1.0.2.0 Production on Mon Feb 5 11:11:33 2018

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

SQL> connect / as sysdba
ERROR:
ORA-12560: TNS:protocol adapter error


SQL> connect sys/password@orcm as sysdba
Connected.
SQL>
SQL>
SQL> archive log list;
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            E:\app\oracle\product\12.1.0\dbhome_1\RDBMS
Oldest online log sequence     6869
Next log sequence to archive   6871
Current log sequence           6871
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect
descriptor


Warning: You are no longer connected to ORACLE.
SQL>

Any assistance would be greatly appreciated.