You playing RP-1 or just Realism Overhaul?
Was looking for that menu that lets you build new launch complexes/simulate launches but I literally cannot find it anymore among all the buttons so I can't test this thing yet lol
Assuming you're playing RP-1, this changed pretty significantly recently. This used to be done in the KSC screen, but the menu to create launch complexes is inside the VAB now.
This is how you manage LCs. I think it is a part of the "Kerbal Construction Time" mod. You can run simulations from the same window