The brew unit is not positioned correctly.
Make sure that the handle is positioned correctly.

Press the button on the bottom of the brew unit handle (1). Whilst doing so, turn the handle forwards (2).
Slide the brew unit into the coffee machine as far as it will go.
CVA 6xxx:
Make sure that the brew unit is pushed over the retainer (3) and clicks into place.
If necessary, push the brew unit in firmly until the brew unit is right up against the housing panel and the drive.

Press the button on the bottom of the brew unit handle (1). Whilst doing so, turn the handle backwards (2).