User is trying to create a new Sequential Mill operation. On the initial operation dialog, you pick the Avoidance Geometry button. On the next dialog, the Clearance Plane button shows 'None' as expected. Pick this button, specify the desired plane, and Ok out.
Back on the main operation dialog, the clearance plane button still says 'None.' You can go back into the Avoidance Geometry and verify the plane you defined, display it, and reinstate it. The clearance plane will not be picked and that button on the main dialog remains at 'None.'
Solution
The cause appears to be the clearance plane parameter in the MCS. If the MCS you are using for this operation has the Clearance Option set to 'Automatic Plane', this seems to cause a conflict with the setting of a new clearance plane in the Sequential Mill operation. This is especially true if you are working in an older part file where the MCS objects are defined with the old style dialog and parameters.
To resolve this, move the operation under a new MCS or change the clearance plane definition in the old MCS to something other than 'automatic.'
Notes