The TVM at my local station only lists the UK-wide railcards, but I have a Scottish Youth Railcard which has the same 1/3 discount on standard off-peak travel as the standard 16-25 railcard. Would it be permissible to select the 16-25 option on the TVM if it's a SYR I would be using with it? The only difficultly I can really see is the difference in peak minimum fare (£8 for SYR, £12 for 16-25).
If the booking office at your origin station is open (assuming it exists), then as your desired ticket is not available from the TVM you should purchase your ticket from the booking office.
The National Rail Conditions of Travel state:
"3. Where the full range of tickets is not available
If you cannot buy an appropriate ticket for the journey you want to make because the
range of tickets that is available at the station from which you intend to start your journey
is restricted, you must buy a ticket or Permit to Travel before you travel that entitles you to make at least part of the journey. You must then, as soon as is reasonably practicable, buyan appropriate ticket to complete your journey. In these circumstances, you only need to pay the fare that you would have paid if you had bought a ticket immediately before your journey. The price you will have to pay will be reduced by the amount paid for the ticket or Permit to Travel."
http://www.nationalrail.co.uk/static/documents/content/NRCOC.pdf
Thus, if only a TVM is available (ie if there is no booking office or the booking office is closed), you should purchase a full fare adult ticket (a 16-25 railcard ticket would not entitle you to make any of the journey as you do not hold said railcard) for part of the journey, and pay the difference in fares to the guard on board the train. Did you have a particular journey in mind?