Your problem is that you are using an unsupported kernel.
Perhaps you can do it in termnal
sudo mhwd-kernel -i linux66 rmc
It that doesn’t work - logout and switch to TTY
First install linux66 - then reboot - at the grub menu ensure your are using linux66 - and switch to TTY
sudo mhwd -r pci video-nvidia
sudo pacman -Syu
sudo mhwd -i pci video-nvidia
While you are in TTY - ensure your system is configure to use early kms