ORA-56747: invalid value ... for parameter sga_target

SQL> alter system set sga_target=4257m;  alter system set sga_target=4257m  *  ERROR at line 1:  ORA-02097: parameter cannot be modified because specified value is invalid  ORA-56747: invalid value 4463788032 for parameter sga_target; must be smaller  than parameter sga_target of the root container

Above error would appear if you were trying to set SGA_TARGET for a PDB and value you are providing is greater than the value set at the CDB (root container) level. Error is self-explanatory; you need to provide a value which is less than the value that you have set at CDB level. 

In above example, SGA_TARGET at CDB level was set to 4256M and when I tried to set sga_target to 4257M in my PDB, error was returned.

 
There are other restrictions as well while you set SGA_TARGET at PDB level. Following is the official document to set this parameter for 19c. 
https://docs.oracle.com/en/database/oracle/oracle-database/19/refrn/SGA_TARGET.html#GUID-4702FDB7-BADC-4997-BC49-1B91088234AE