DIAGNOSIS The single-serve side isn't being recognized because the reservoir switch isn't engaged. You likely have the carafe station's drip tray or the reservoir in the wrong position.
FIX Look at the right side of the unit where the carafe station slides in. You must have the 12-cup glass carafe placed there, sitting perfectly on the warming plate. The brewer uses a sensor in that spot to know the carafe is present before it allows single-serve brewing. Make sure the carafe is fully seated and the drip tray underneath it is clicked all the way in.
IF THAT DIDN'T WORK Check the reservoir itself. It must be pushed firmly and completely back into its slot against the main unit. Listen for a solid click. If it's even a quarter inch out, the internal micro-switch won't trip.
HEADS UP Never run the single-serve side without a travel mug or cup in place. The hot water dispenser has no safety shutoff and will spray boiling water.