Hi guys, I sure need some help trying to fix this issue. I recently bought a 1080 Ti used for $525 (eBay) when I installed it, my monitor didn't recognize it so that I would get [no signal] message. The only way I could enter (see anything) in the OS was through the motherboard integrated graphics.
I tried several solutions I found on youtube, like disabling Intel Display adapter in the Device Manager option, to no avail. After that, I did a CMOS reset to the motherboard, switch the DisplayPort to the 1080Ti, and my monitor finally picked up the signal, and I could enter the OS as usual. Then I started having screen flickering, more like the screen would turn OFF and ON continuously.
I went back to the bios thinking I should switch my primary display to PCIe and on my System Agent (SA) Configuration > Graphics Configuration > Primary Display when I try to choose I only get three options [Auto] [IGFX] [PEG], but PCIe is not available. I don't understand why.
Now I've switched back to my 980 Ti, and there is no flickering problem, but just out of curiosity I went back into the bios and the option for PCIe it's still not available.

Note 1: each time I switch GPUs I delete all previous drivers with DDU.

Note 2: I reverted everything to original clock settings, so my CPU, RAM, and GPU are running with no overclock whatsoever.

I'm not an expert I just did some research so if anyone can enlight me to why this is happening it would be amazing.

My System Specks:
MONITOR: Acer Predator X34 (Using DisplayPort)
CPU: Intel Core i7 6700K
MOTHERBOARD: Asus Z170 Deluxe
RAM: Corsair Dominator DDR4 32G
PSU: EVGA Supernova 850 P2
GPU: GTX 980 Ti Gigabyte G1 Gaming (currently installed) and EVGA GTX 1080 Ti SC2 Gaming (the one with the issues)

Thank you for the clarification Sajin, I freaked out because on my motherboard manual say's PCI-E option should be available. I contacted the eBay seller asking, and he said he tested the GPU before the sell.
Right now I just reset everything in bios back to default (again), went into the OS in Safe Mode, deleted the drivers with DDU > turned OFF the PC > Installed the 1080 Ti > Changed the DisplayPort cables. When I turn the PC ON I got signal. I don't know if the problem is fixed or not, but no flickering so far. All the latest drivers are installed. I will update if the problem persists.

My previous DisplayPort wasn't VESA Certified. This new one is. Either way, I plan to do precisely that. I will be looping Unigine Valley benchmark and see if the problem re-appears.