There are two possible causes:

An attempt was made to issue an ALTER SESSION statement with an invalid NLS parameter or value.

The NLS_LANG environment variable contains an invalid language, territory, or character set.

Fix:

Unset the NLS_LANG environment variable

Windows - The NLS_LANG must be unset in the Windows registry (re-named is best). Look for the NLS_LANG subkey in the registry at \HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE, and rename it.

Linux/UNIX - Here you simply issue the Linux command “unset NLS_LANG”