The reason the you're receiving this error is due to Oracle's parameters for SHARED_POOL_SIZE. Note that when SGA_TARGET is set and the parameter is not specified, then the default is 0 (internally determined by the Oracle Database), but if the parameter is specified, then your specified value indicates a minimum value for the memory pool.
In the case where a value was set for SGA_TARGET, that would be the value you would need to update rather than the SHARED_POOL_SIZE since by setting SGA_TARGET, you are using automatic SGA management. Hence, there is no need to manually set the value of SHARED_POOL_SIZE because Oracle will internally transfer memory between the SGA components.
In the case where you are more concerned with setting a larger value for SGA_TARGET, you can also make a larger value for SHARED_POOL_SIZE but the value must be smaller than SGA_TARGET to avoid encountering the following issue:
SGA_TARGET = 1GB
SHARED_POOL_SIZE = no value
You will encounter an issue when the value of SHARED_POOL_SIZE exceeds the value of SGA_TARGET.
It is recommended to set the SGA_TARGET value at a minimum of 5GB. Therefore, if the SHARED_POOL_SIZE value is at 1GB, the SGA_TARGET will still have at least 4GB for allocation of other memory components that are concurrently stored in SGA_TARGET.
NOTE: Be sure to restart the Oracle Database after making the described value changes. For additional information, refer to SHARED_POOL_SIZE or SGA_TARGET.