I'm trying to move an entire model using MoveTo:(), but I don't know how to set the PrimaryPart of it, because it's currently set to nil.
The easiest way to do this, is to select the model and in the Properties viewer, select "PrimaryPart" and then select the part you wish to be the primary. An easy way to move it after this is
workspace.Model:SetPrimaryPartCFrame(CFrame.new(0 0, 0)); -- The 0, 0, 0 can be any Vector3 coordinates you desire and it will more the model with the primary part where you position it with this command.
Hope this helps!
Using that guys code^^ But this is to set the part, not the C-Frame
game.Workspace.Model:SetPrimaryPart(Part name here)