You need to check the sources.list for the non-free Debian Repositories - if you are using the Gnome desktop
Code:
sudo gedit /etc/apt/sources.list
If you are using a different desktop then replace gedit with your text editor that you are using. - now look for the following
deb
http://deb.debian.org/debian buster main contrib non-free
#deb-src
http://deb.debian.org/debian buster main contrib non-free
deb
http://deb.debian.org/debian-security/ buster/updates main contrib non-free
#deb-src
http://deb.debian.org/debian-security/ buster/updates main contrib non-free
deb
http://deb.debian.org/debian buster-updates main contrib non-free
#deb-src
http://deb.debian.org/debian buster-updates main contrib non-free
If they are not there then add them and save the sources.list file
Then run
Code:
sudo apt install firmware-iwlwifi
now reboot