Verify the solid bodies and their materials.
In the history tree, expand Model / 1 / Solids.
Under Solids, branches for copper, steel_1010, and vacuum solid materials are visible.
Expand each branch to see the names of the bodies made of that material. When you click a body's name in the tree, it is highlighted in the modeler window.
Note the following bodies, which will be involved in the co-simulation:
Under copper, there are seven coils, named Coil_1 through Coil_7.
Under steel_1010, there is a single core body named Core.