In this case, the issue is likely for one of two reasons:
1. You may be using the wrong access token and attempting to access a list ID that belongs to another account.
2. The list no longer exists. It could have been deleted through the API or from the UI.
You can investigate both of these possibilities by making a GET request to /v2/lists and searching to make sure that the list ID you are using is present. If the list ID is present, then you should be able to fetch the details.