ERROR: ORA-01031: insufficient privileges After installing Oracle and opening Command Prompt, I typed in 
sqlplus "/as sysdba" 
and got an error message 
ERROR:
ORA-01031: insufficient privileges
and then it asks me to enter the user-name.
I'm using my own personal computer and although the name given after C:\Users\ is not called "Administrator", it's my username when I used to log in to this computer and since I'm the only one using the computer I'm assuming it's the same as "Administrator" and when I opened account settings it says administrator under my account name.

Microsoft Windows [Version 10.0.17134.228]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\Users\linus&gt;sqlplus "/as sysdba"

SQL*Plus: Release 11.2.0.1.0 Production on Mon Aug 20 10:56:53 2018

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

ERROR:
ORA-01031: insufficient privileges


Enter user-name:


What might be the cause of this and what can I do to solve it? Thanks!
since I'm the only one using the computer I'm assuming it's the same as "Administrator"


That's not enough to be an Administrator. 

Make sure that your user belongs to ORA_DBA OR ORA_&lt;%ORACLE_SID%&gt;_DBA OS group on Windows. 

As well check the value set for SQLNET.AUTHENTICATION_SERVICES parameter on sqlnet.ora(%ORACLE_HOME%\network\admin) file. NTS should be included in the value. 

SQLNET.AUTHENTICATION_SERVICES = (NTS)