I'm getting 'The device cannot find enough free resources that it can use (Code 12).

I've got:-
ASRock H81 Pro BTC motherboard
4Gb RAM
2 x Gigabyte GeForce GTX 1080Ti
Windows 10

(1st card connected to 16x slot directly on motherboard, 2nd via pcie riser)

I did a quick search on Google and it was saying to change the TOLUD setting to 3.5G but I can't find that anywhere in my bios. If I disable the first card and reboot then the second one works fine and vice versa. I set the PCIE to Gen1 whatever that means too.

Anybody got any ideas please?