The PCIe 3.0 x16_2 slot on the ASUS Z170-P D3 motherboard only has four PCI Express lanes wired to it. NVIDIA graphics cards usually require at least eight PCI Express lanes to function. AMD graphics cards will function with a minimum of four PCI Express lanes.
Okay so if i want to use SLI can i then buy another ASUS Strix GeForce GTX 960 and it will all be fine. Like no need to upgrade motherboard?
You'll only need to upgrade the moherboard if it isn't SLI certified, and you'll need the exact same model of card, a blower style Founders Edition, most likely, wont work with any other card even if they are both 960's. Crossfire will though, you can even mix and match cards like an R9 280x (If memory serves me right) And a Radeon HD 7970 since they are both Tahiti based cores, based on the famous GCN architecture from AMD, and same with other cards that have cores based on the same architecture (This only works with Crossfire, not SLI)