With the mistake fixed, the F vs D curve flattened out.
The response also looks cleaner and I forgot to include any damping in the model, so the result is impressive.
To be effective in your internship, you need to develop geometry creation and editing skills, model building skills, and simulation debugging skills. I gave you the 3D surface model of the two springs of a half cell in the attachment (Geometry Options for Spring) in my August 7 reply on P2 of this discussion. I have attached just the geometry here for you in a 7z file below.
Use the Mirror button to reflect surfaces about the center of the unit cell then use the Move tool, holding the Ctrl key to make a copy to stack up 3 layers. Instead of the Move tool you can also use the Linear Pattern tool to copy them down. You will have to sketch out some lines for the Frame and extrude those out into surfaces. I'll check in later to see how it's going.