When I create a new ticket in the support center and click the ‘Affected tenant’ dropdown the tenant I want to choose is not present.
It’s most likely that the correct subscription is not selected. Some customers have multiple subscriptions and may have production tenants under one and development tenants under another, for example.
It may be that the tenant you wish to select is under a different subscription. You can change the subscription selected using the dropdown at the top of the page: