Possible missing firmware /lib/firmware/rtl_nic/… If this alert occurs in your Debian, it can be quick solved by installing realtek-firmware package.
The message that appears on the screen is similar to this.
... update-initramfs: Generating /boot/initrd.img-4.19.0-5-amd64 W: Possible missing firmware /lib/firmware/rtl_nic/rtl8107e-2.fw for module r8169 ...
Check if your /etc/apt/sources.list includes below line.
deb http://httpredir.debian.org/debian/ buster main contrib non-free
If not, add above line using editor of your choice (nano/vim/kate) or just type below command in terminal.
# echo "deb http://httpredir.debian.org/debian/ buster main contrib non-free" | \ # tee -a /etc/apt/sources.list
Update the list of available packages and install non-free firmware-realtek package.
# apt update && apt install firmware-realtek
Alternative path for Ubuntu
There is no firmware-realtek package in Ubuntu. Instead, try to install linux-firmware or linux-firmware-nonfree.