As of this posting, there appears to be an issue with Zwift that is causing the rolldown calibration of a Hammer or Magnus to fail. This is not an issue with your Hammer or Magnus trainer. Zwift is hard at work to resolve this.
Until a fix has been completed, we recommend that a rolldown calibration be done using the Rouvy software (no subscription required). There are support articles on Zwift and Rouvy to walk you through this process on Rouvy.
It is completely fine to calibrate your trainer in Rouvy and then ride in Zwift because the calibration of the trainer is stored within the trainer; not the software.