I am trying to purchase train tickets for May 13, 2020 to go from Budapest to Vienna but am unable to do so through DB Bahn. What am I doing wrong?
Too early, I believe. Check the Seat61.com website for information on when tickets typically go on sale.
Yes, way too early! Try again in february. Although I think you will get the best price buying from MAV or ÖBB.
Way way too early, and the wrong company.
Times for next May are not available yet. In most cases ticket sales start 3 months before departure, The cheapest tickets are 1-3 months in advance, after that the prices start to go up.
DB = Deutsche Bahn = German Railways. As the name suggests, runs trains in Germany and only sell tickets for trains within, to or from Germany.
You need to buy from MAV (Hungarian Railways), or ÖBB (Austrian Railways), but not until next Feb/March.
For all the info you need about this route, see here: https://www.seat61.com/trains-and-routes/vienna-to-budapest-by-train.htm
You probably will do better if you purchase your tickets from the company that will be operating the train
Ticket Site: https://www.mavcsoport.hu/en
And yes its too early. They book through the end of the year right now. I think its because they havent loaded the fares for next year yet. But you can see the schedules and prices for planning, same trains run every day.