Not able to set NLS_Lang

SQL> SELECT USERENV('LANGUAGE') "Language" FROM DUAL;

Language

----------------------------------------------------

AMERICAN_AMERICA.AL32UTF8

SQL> alter session set nls_language=&language;

Enter value for language: AMERICAN_AMERICA.AL32UTF8

old 1: alter session set nls_language=&language

new 1: alter session set nls_language=AMERICAN_AMERICA.AL32UTF8

alter session set nls_language=AMERICAN_AMERICA.AL32UTF8

*

ERROR at line 1:

ORA-00922: missing or invalid option

SQL> alter session set nls_language='&language';

Enter value for language: AMERICAN_AMERICA.AL32UTF8

old 1: alter session set nls_language='&language'

new 1: alter session set nls_language='AMERICAN_AMERICA.AL32UTF8'

ERROR:

ORA-12705: Cannot access NLS data files or invalid environment specified