Error:"Variables property does not include all degrees of freedom in xmesh".

Hello,
I have tried to simulate two groups of cylinder permanent magnets and try to calculate the magnetic forces.
I use Swept mesh for the magnets and Free Quad for the air (which I use a big sphere) around the magnets.
Now the problem is: when I run it, there is the error"Variables property does not include all degrees of freedom in xmesh" .
Could anybody tell me what could be the problem?

Thanks a lot!

Cui