The driver is available from the AUR (Arch User repository). Make certain that you have the appropriate Linux headers for your kernel and dkms and base-devel package installed as well.
Note: from the AUR page: You may have better luck with the following driver:
To all having an issue with this driver:
Please try this alternate package:
https://aur.archlinux.org/packages/rtw88-dkms-git.
Install via Pamac GUI or in terminal:
pamac build rtl8821au-dkms-git
OR
pamac build rtw88-dkms-git
Reboot
Please read these as well: