I am pretty confident that, if you arrive in Basel on time, you will be able to take that train. All you need is a valid ticket (or a pass). You do not need a reservation, and you will not be turned away as there is nobody to turn you away, or to prevent you from getting on the train.
If you are coming from Germany it is very well possible that your train will be late. In that case, just take whatever the next train to Spiez is…