I have been to my HR and nobody wants to help me with this question. I have also looked through some Codes and can't seem to find anything.

Here is the situation. One person is due to retire in a month. To this point there was no mention of filling that position or nothing said that that position was available. All of the sudden we get an email from the Manager that the position is being filled by another person in the department. Is this legal? Is the employer required to post the position within the company before it is filled?

If someone knows I would appreciate some advice.

Unless you are under a bona fide contract or CBA that says otherwise, the employer is NOT required by law to post open positions. It's their choice whether to do so or not.