Tuning ASSM or ASM target can never be done within SCOPE=MEMORY or even SCOPE=BOTH, which is not allowed.
SQL> alter system set sga_target=64g scope=both;
alter system set sga_target=64g scope=both
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00823: Specified value of sga_target greater than sga_max_size
Did you have ever set SGA_MAX_SIZE? Let's make sure this by inspecting pfile.
SQL> create pfile from spfile;
File created.
If there's no SGA_MAX_SIZE in pfile, you can change it in spfile safely within spfile scope.
SQL> alter system set sga_target=64g scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 6.8413E+10 bytes
Fixed Size 2270360 bytes
Variable Size 3087010664 bytes
Database Buffers 6.5230E+10 bytes
Redo Buffers 93585408 bytes
Database mounted.
Database opened.
SQL> show parameter sga_
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
sga_max_size big integer 64G
sga_target big integer 64G
If there's SGA_MAX_SIZE set in pfile, you should raise SGA_MAX_SIZE first, then raise SGA_TARGET. Of course, within SCOPE=SPFILE.