ORA-1031 while connect as sysdba on windows platform

Two possible causes are:

1. SQLNET.AUTHENTICATION_SERVICES = (NTS)

The value is not NTS . By default in 8.1.x and onwards, a new database uses Windows Native Authentication by having the above setting in the "sqlnet.ora" file.

2. No appropriate DBA group

Create group ORA_DBA or ORA__DBA, and add your windows ID to it.