If you are connecting to The Tacx app and it is not working properly to control the power, then it’s a problem with the trainer. Contact Tacx. Hopefully they will help you.
Frustrating, I understand. Tacx will sort it out I’m sure.
In the meantime, if you have a regular turbo trainer, a speed sensor and a cadence meter, you can use virtual power in TR or Zpower in Zwift and still do the interval workouts effectively. These tools reverse calculate your power from speed (and trainer model) and work great.
Good hunting.