Alter session not set in pl/sql procedure

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SQL

While writing procedure n pl/sql developer, alter session not working. Can't set alter session.Please find the below code and let me know any idea.

BEGIN
              SELECT ls_live_schema||'_arc' INTO ls_from_user FROM dual;
              dbms_output.put_line(ls_live_schema);
              ls_sql:='Alter session set current_schema='||ls_from_user;
                EXECUTE IMMEDIATE ls_sql;
                -- ' || '' ||
                        --  ls_live_schema || '' || '_ARC';
                          --commit;
                          SELECT sys_context('USERENV', 'CURRENT_SCHEMA') INTO ls_curr_user FROM dual;
         --     using ls_from_user;
END;