You need a Ref_Node, in physics we call that a reference-ground-plane, for the E-fields that gets decoupled to this node.
There are few small , however important things, once understood and taken care of - rest is very easy.
From layout perspective the substrate - is the reference node.
If you are using Net-Name-Space "Layout" - use the node name - of your omnipresent substrate layer.
How to know? - what node-name did my substrate get?
- Use LVS probing
- you can check extracted Layout-netlist - and see what is the ground netname.
If you are using Schematic as input net name space, use schematic netname.
Don't mix.