I dont have anything else in other slot, the problem is that I cant put the gpu in the first slot (cpu cooler noctua nh-d14 too big...)
Is there a way to enable x16 in the bios?
That's it. Only one slot gets you the X16 unless you get one of those mega-motherboards with four PCI-E slots.
I saw a review where the AMD video cards are virtually uneffected by X16 or X8, it was only at X4 that they lost a few percent. The Nvidia were not so good, they lost a few percent at X8 and more at X4. So you are only losing maybe one or two FPS. Don't worry about it.