You're using a clear glass mug. The optical cup sensor can't detect it.
Fix: Immediately swap the glass mug for a ceramic or opaque cup. The "PLACE CUP" light will stop flashing and you can brew.
If that didn't work: Ensure the cup is placed directly on the drip tray plate, not on the raised edges of the drip tray assembly. Also, make sure the drip tray assembly is fully seated and not removed.
Heads up: The drip tray is designed to accumulate liquids. Slide it out toward you (keeping it level) periodically to empty and clean it with a damp, soapy cloth. Do not put it in the dishwasher.