As @Nachlese said, to install/compile something like this from the AUR, there are quite some prerequisites.
Firstly, you need the kernel headers for your installed kernel(s). You also need the dkms package. Then you can go ahead.
Installing headers for currently installed kernels
pamac install $(pamac list --installed --quiet | grep "^linux" | grep "^linux[0-9]*[-rt]*$" | awk '{print $1"-headers"}' ORS=' ')
Then you also need at least thee dkms package, but that sh0ould be installed when building the drivers, IIRC.
The drivers themselves are in the AUR:
$ pamac search 8812au
[...]
rtl8812au-dkms-git 5.13.6.r129.g0fe44f8-1 AUR
rtl8812AU chipset driver with firmware v5.13.6
[...]
So to build them, theoretically you need to run:
pamac build rtl8812au-dkms-git
Hope this helps!
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