This error means that the door is not locked properly. Laundry may be caught in the door. Open and close the door again and press the "start reload" button (play and pause button). If necessary, open the washing machine door, remove some of the clothing items and then push the door closed again. You can also try switching the washing machine off and on again, setting the programme and then starting the programme.
If the issue persists, we advise against attempting repairs yourself. Instead, book a repair with one of our highly trained and qualified Bosch engineers.