Tohoma is correct. There a lot of parameters in HFSS you need to satisfy before HFSS is happy.
1. Specify all your ports
2. Select within your model the garnets or ferrite and specify it's characters.
3. Once you've done that you should be able to select magnetic bais. If you're running HFSS alone without Maxwell, you will need to select a uniform field. If you have Maxwell, would select non uniform bias and point Maxwell.
My experience HFSS is accurate for waveguide units but bad for coaxial or drop-in circulators / Isolators.