i have a fairly decent machine with a GTX 1070 8GB card but if i try to start the game in UE i get this error. 
Do you get good fps on high quality in other games? (You should, I have GTX1070 and get about 120 fps in Fortnite.) I had an issue once where when I downloaded some new drivers for NVIDIA Geforce Experience, my computer (this was a different one with a 1050ti) started trying to use the CPU for games instead of the actual graphics card and many games wouldn’t even load.
Make sure the game is running on the right card, assuming you also have an integrated card.
both my monitors are plugged in my GPU and all other games i get expected performance