/tftpboot is tftp root diretory.

/tftpboot$ ls -lR ubuntu-20.04-desktop-amd64
ubuntu-20.04-desktop-amd64:
-r--r--r-- 1 root root 89971296  5月 10 22:45 initrd
-r--r--r-- 1 root root 11657976  5月 10 22:45 vmlinuz

Note:
The initrd and vmlinuz are copied from the Ubuntu 20.04 installer disc image /casper/{initrd|vmlinuz}.

/tftpboot$ ls -lR linuxmix
linuxmix:
-rw-r--r-- 1 root root   256  3月 28  2012 japanese.kbd
-r-xr--r-- 1 root root  5796  3月 28  2012 kbdmap.c32
-r-xr--r-- 1 root root 57652  3月 25  2012 menu.c32
-r-xr--r-- 1 root root 16466  3月 25  2012 pxelinux.0 
dr-xr-xr-x 2 root root  4096  3月 23  2017 pxelinux.cfg

linuxmix/pxelinux.cfg:
-rw-r--r-- 1 root root 2647  5月 11 00:12 default

Note:
Configure dhcpd.conf entry as follows

  filename "linuxmix/pxelinux.0";

/tftpboot$ cat linuxmix/pxelinux.cfg/default
default menu.c32

MENU TITLE Network boot systems
PROMPT 0
ALLOWOPTIONS 1
NOESCAPE 0
TIMEOUT 600
KBDMAP japanese.kbd

LABEL Ubuntu 20.04 x64 Desktop
	KERNEL ../ubuntu-20.04-desktop-amd64/vmlinuz
	APPEND ip=dhcp boot=casper netboot=nfs nfsroot=192.168.0.160:/export/ubuntu-20.04-desktop-amd64 initrd=../ubuntu-20.04-desktop-amd64/initrd vga=791 nomodeset --

Note: default file settings.
Add "ip=dhcp" to the APPEND line. The Ubuntu 20.04 doesn't keep IP address configured by PXE boot loader. So reconfigure the network settings.

The IP address 192.168.0.160 is the NFS server address. The NFS server shares /export/ubuntu-20.04-desktop-amd64 which is mounted "ubuntu-20.04-desktop-amd64.iso" image.

Remove "quiet" and "splash" from the APPEND line. Add "vga=791" and "nomodeset" to the APPEND line. The Ubuntu 20.04 does many check steps on booting process. To confirm helthy booting by seeing what is going on.