How do I contact Apple customer support regarding refund not credited on my account?

You can certainly contact Apple anytime you want. Support options will vary by region:

Contact Apple Support - Apple Support

As you can see in the link provided by Allan Jones:

If your refund is approved, the funds are applied to the same payment method that you used to buy the item. Here's how long it takes for a refund to appear on your account or statement:

• Store credit—It might take up to 48 hours to see the refund in your Apple Account balance.
• Mobile phone billing—It might take up to 60 days for the statement to show the refund. Processing time might be affected by your carrier. You can contact your carrier for an update.
• Credit and debit cards, Apple Pay, Apple Cash, and all other payment methods—It might take up to 30 days for the statement to show the refund. If you don't see the refund after 30 days, contact your financial institution.

So, if it has been approved and the purchase was made with a credit card, then you are instructed to contact the financial institution if it has been longer than 30 day. In my experience, a refund from any source will be posted to my credit card in 5-7 business days.

Apple does not respond in these forums. Everyone here is an end user like you and we cannot research your issue.

Please start here:

Check the status of a refund for apps or content that you bought from Apple - Apple Support