Payment gateway settings: Verify that the payment gateway you are using supports the Cash on Delivery option. Not all payment gateways offer COD as a payment method. Check your payment gateway settings to ensure COD is enabled and properly configured.
Shipping zone restrictions: COD may be limited to specific shipping zones or countries. Double-check your shipping settings to ensure that the COD option is available for the applicable zones. If necessary, adjust your shipping settings to include the desired zones.
Theme or app conflicts: Sometimes, conflicts between your theme or installed apps can interfere with the COD option. Try temporarily switching to a default Shopify theme to see if the issue persists. If the problem is resolved, you can narrow down the conflicting theme or app by reactivating them one by one.