Setting up SQL Server® Authentication for SQL Server 2012 or later (4039483)

Beginning with SQL Server 2012, security privileges changed and you cannot use the LocalSystem account. Previously, you could use the LocalSystem account as the default for the sysadmin role, use authentication for SQL Server 2012 or later, use a domain account (including Administrator) that has the SQL Server sysadmin role or change the Log on as option for SQL Server Service to use a domain user that has the required privileges.

If the SQL Server is not part of a domain, you can use a local user that has the sysadmin role or change the Log on as option for SQL Server Service to use a local user that has the required privileges,