How can install the libc properly?

The package libc can't be installed directly with opkg install.

root@OpenWrt:~# opkg update
root@OpenWrt:~# opkg install  libc
Unknown package 'libc'.
Collected errors:
 * opkg_install_cmd: Cannot install package libc.

So i have to install it manually.

root@OpenWrt:~#     wget
--2021-06-18 02:35:32--
Resolving 2a01:4f8:251:321::2,
Connecting to|2a01:4f8:251:321::2|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 249368 (244K) [application/octet-stream]
Saving to: 'libc_1.1.24-2_x86_64.ipk.2'

libc_1.1.24-2_x86_6 100%[===================>] 243.52K   266KB/s    in 0.9s    

2021-06-18 02:35:35 (266 KB/s) - 'libc_1.1.24-2_x86_64.ipk.2' saved [249368/249368]

root@OpenWrt:~# opkg install libc_1.1.24-2_x86_64.ipk
Installing libc (1.1.24-2) to root...
Configuring libc.

Why can't locate it ?

root@OpenWrt:~# which libc
root@OpenWrt:~# find / -name 'libc'

It's rather simple, you can't. Neither kernel, nor libc are packaged (and just provide empty meta-packages for dependency resolutions); sysupgrade to a newer image build.