DIAGNOSIS This is often a simple but frustrating one: the machine thinks the brew group isn't fully locked into place. There's a safety microswitch that prevents operation if it's not correctly seated.
FIX 1) Make sure the machine is on and warmed up. 2) Open the front service door. 3) Pull the brew group release lever to eject the brew group. 4) Inspect it. Make sure there's no old coffee puck stuck in it and that it's clean. 5. Reinsert it firmly. You should feel a distinct click and see it sit flush. Sometimes you need to give it a little extra push at the end of its travel. Close the service door and try the brew function again.
IF THAT DIDN'T WORK If it's definitely seated correctly, the microswitch itself might be faulty or have a loose connection. You can try unplugging the machine for a few minutes to reset the electronics. If the problem is consistent, accessing and checking that switch involves taking off the outer casing, which is a more advanced repair.
HEADS UP Always remove and rinse the brew group regularly. If coffee grounds and oils build up on its sides or the tracks it slides on, it can prevent it from sliding all the way home, triggering this exact problem. A quick weekly rinse prevents this.