Your database instance is started with a PFILE.
You want to increase the size of the buffer cache. Free memory is available to increase the size of the buffer
You execute the command:
SQL> ALTER SYSTEM SET DB_CACHE_SIZE=1024M;
Which is the outcome?
Change is applied to the current instance, but does not persist after instance restart.
The value is changed only in the PFILE and takes effect at the next instance startup.
The value is changed for the current instance and in the PFILE.
It fails because the SCOPE clause is missing.