The link in
https://github.com/CoreELEC/CoreELEC/tree/coreelec-20/projects/Amlogic-ce/packages/linux-drivers/RTL8821CU/package.mk
is dead. https://github.com/morrownr/8821cu-20210118/ does no longer exist. It seems that GitHub - morrownr/8821cu-20210916: Linux Driver for USB WiFi Adapters that are based on the RTL8811CU, RTL8821CU, RTL8821CUH and RTL8731AU Chipsets - v5.12.0.4 is now the current git.