Either you posted the wrong query or you copy/pasted the wrong error.
2 chk_time number;
4 chk_time := 600;
5 select to_char(to_date(chk_time),'sssss'),'hh24:mi:ss') from dual;
ERROR at line 6:
ORA-06550: line 5, column 55:
PL/SQL: ORA-00923: FROM keyword not found where expected
ORA-06550: line 5, column 1:
PL/SQL: SQL Statement ignored
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.