Compile Linux Kernel ,RTAI and LinuxCNC from Source Code

yes! I using the NTULINUX version. I don't know why？

The NTULINUX version is better-integrated with LinuxCNC. I would stick with it.
But you might need to install musl (and maybe musl-dev)
packages.debian.org/source/buster/musl

Do I need to install musl (and maybe musl-dev)
packages? debian.org/source/buster/musl
This is the option that needs to be set when compiling RTAI? After I make menuconfig, there is no function library set in it. Which option is it?

I think that you should just need to install musl and musl-dev
(sudo apt-get install musl musl-dev)

That might make the error message go away. Or you might need to recompile RTAI.