train to Salzburg -- no seat reservation on some trains?

Regional trains dont have reserved seats. Arrive a bit early and board as soon as possible, and grab the first available seat, and you shouldn't have a problem.

If your regional train ORIGINATES in Munich, it will be EMPTY when the doors first open to passengers from the platform.

EC 217 goes from Saarbrücken, Germany to Graz, Austria, the only train to make such a long trip and go between Munich and Salzburg all morning. That might be why it sells out; not just Munich to Salzburg riders.

You won't have a problem getting a seat on the RE train.

Sep 2024 is ages away. So far away in fact that the railways probably do not even know yet with which consist they will run the EC 217.
So as a result the seat inventory has not been loaded yet. And because the booking system has been programmed by people who cannot imagine that someone would be booking trains for September already now the system does not deal with this situation well.

So the system tries to find a seat for you, doesn't find one, and throws an error. And since it cannot distinguish between "there are no seats because they are all gone" and "there are no seats because they have not been loaded yet" the message can be a bit confusing.

Here is the thing: If trains sold out weeks in advance they would be useless. Especially in Germany people would not accept that. There would be hell to pay. DB management would be sacked. The Verkehrsminister would have to answer questions in parliament.

So if you are looking at trains more than a few months in the future, and getting odd results, it is just an artefact of you dealing with a system that does not expect people to buy tickets, and reserve seats that far in advance.

So do not worry. You will be able to reserve seats for EC 217 closer to your travel date.

thanks all for the advice!

I wouldn't bother with EC217. Long-distance trains in Germany have a significant risk of delay, whereas regional trains still perform quite well. The numerous BRB (slowest option but good with Bayern ticket), Railjet or Westbahn trains are the way to go IMO.

You can use the fact that EC271 has a risk of delay to your advantage. Buy a very cheap Supersparpreis Europa in advance. See how it becomes a Flexpreis ticket as soon as the train is significantly delayed...