ORA-06502: PL/SQL: numeric or value error%s
 *Cause: An arithmetic, numeric, string, conversion, or constraint error
         occurred. For example, this error occurs if an attempt is made to
         assign the value NULL to a variable declared NOT NULL, or if an
         attempt is made to assign an integer larger than 99 to a variable
         declared NUMBER(2).
 *Action: Change the data, how it is manipulated, or how it is declared so
          that values do not violate constraints.

ORA-06502: PL/SQL: numeric or value error%s
*Cause: An arithmetic, numeric, string, conversion, or constraint error
occurred. For example, this error occurs if an attempt is made to
assign the value NULL to a variable declared NOT NULL, or if an
attempt is made to assign an integer larger than 99 to a variable
declared NUMBER(2).
*Action: Change the data, how it is manipulated, or how it is declared so
that values do not violate constraints.

DECLARE
    v_dummy NUMBER;
    v_value VARCHAR2(1); -->Check This 
BEGIN
    v_value := Substr(:BB_MM_INFRA_CHECKLIST.checklist_description, 1, 1);

    IF NOT (( Ascii(v_value) >= 65 AND Ascii(v_value) <= 122 )) THEN
      v_dummy := Func_alert('Required_Alert', '#soochanaa','pahlaa akshar sankhyaa naheeM ho sakataa.');

      RAISE form_trigger_failure;
    END IF;
END;  

v_dummy := func_alert('Required_Alert', '#soochanaa', 'pahlaa akshar sankhyaa naheeM ho sakataa.');

#soochanaa', 'pahlaa akshar sankhyaa naheeM ho sakataa.');