This happens when a transaction continues in Cancelling status indefinitely, preventing it from being cancelled and, consequently, preventing a request in OMS. This happens when VTEX is awaiting a response from the gateway system, but the correct response is not sent. We can see in the picture below an example of a gateway response preventing VTEX from proceeding with the cancellation process:

To solve this, you must contact the gateway responsible for payment, give the transaction details and request cancellation.