On Oracle 12c. is there any way to enable the audit trail on SELECT on whole schema of some users?
Please try the following:

AUDIT SELECT TABLE, INSERT TABLE, DELETE TABLE, EXECUTE PROCEDURE
ON &lt;Schema_name&gt;
BY ACCESS;


Using Oracle fine grain auditing we can achieve auditing of all statements of entire schema:  

begin
    for c1 in (select * from all_tables where owner = '&amp;schema') loop
        dbms_fga.add_policy (
            object_schema   =&gt; c1.owner,
            object_name     =&gt; c1.table_name,
            statement_types =&gt; 'SELECT,UPDATE,DELETE,INSERT',
            policy_name     =&gt; c1.table_name
        );
    end loop;
end;
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