To fix the “firmware: failed to load iwlwifi-8265-36.ucode” error when configuring Intel WiFi on Debian Linux, you will need to install the firmware-iwlwifi package.
Try running the following command as root:
apt-get update && apt-get install firmware-iwlwifi
Then reboot the system.
Alternatively, you can download the package from the official Debian package repository and install it manually using dpkg:
wget http://ftp.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-iwlwifi_20210114-1_all.deb
dpkg -i firmware-iwlwifi_20210114-1_all.deb
After that, reboot the system again, and the error should be resolved.
If the problem persist, please check the version of the firmware that you are trying to load, this version might not be compatible with your hardware or kernel. If this is the case, you might have to install a different version of the firmware or update your kernel.