SQL> alter system set  sga_max_size=2048M scope=spfile;
alter system set  sga_max_size=2048M scope=spfile
*
ERROR at line 1:
ORA-32001: write to SPFILE requested but no SPFILE specified at startup

SQL> show parameter spfile;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
spfile                               string

SQL> create spfile from pfile;

File created.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.

Total System Global Area 1068937216 bytes
Fixed Size                  2166536 bytes
Variable Size             427819256 bytes
Database Buffers          624951296 bytes
Redo Buffers               14000128 bytes
Database mounted.
Database opened.
SQL> alter system set sga_max_size=2048M scope=spfile;

System altered.

SQL> alter system set sga_target=2048M scope=spfile;

System altered.

SQL>  shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.

Total System Global Area 2137886720 bytes
Fixed Size                  2161400 bytes
Variable Size             469763336 bytes
Database Buffers         1644167168 bytes
Redo Buffers               21794816 bytes
Database mounted.
Database opened.
SQL> show parameter sga;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     FALSE
sga_max_size                         big integer 2G
sga_target                           big integer 2G

ORA-32001: write to SPFILE requested but no SPFILE specified at startup