Diagnosis
The USB-C port on the travel case is finicky and isn't receiving power, or the case's internal battery is fully depleted and needs a longer initial charge.
How to Fix It
Reseat the USB-C cable firmly. Unplug the cable from both the case and the wall adapter. Plug the USB-C end into the travel case first, ensuring it clicks in fully. Then plug the other end into a high-power USB wall adapter (at least 2 amps is best). Charge the empty case for at least 3 hours before testing.
If That Didn't Work
The case itself might need a reset. With the USB-C cable plugged in, press and hold the small button on the side of the case for 10 seconds. Release, then unplug and replug the cable to restart charging.
Heads Up
The travel case will only charge the brush if the brush's own battery is below 75%. It's designed for top-ups, not for charging from dead.