Probably due to kernel 5.8. AUR package page comment says:
so it’s likely that it’s failing to compile.
Looks like there is a fork which fixes compilation against 5.8:
This will need you to temporarily set a different upstream for the PKGBUILD:
-source=("git+https://github.com/brektrou/rtl8821CU"
+source=("git+https://github.com/mbrysa/rtl8821CU"
then build again.