Port 1 does not have a solved inside material on either side

hello everybody!
i am beginner in the use of HFSS. I design pyramidal horn antenna, validate is ok.but error after change material to aluminium.
what's wrong with the project? please somebody help me!

Sounds like you changed the wrong material to aluminium. Make sure the dielectric inside the waveguide feed wasn't changed.

thanks PlanarMaterial, how make the dieletric inside the waveguide feed isn't change? please tell me step by step.help me!

1) Select the object.
2) Observe the assigned material.
3) If the material is aluminium, change it to a dielectric.