I am currently running elementaryOS 7 on an old MacBook Air (MacBookAir6,2) of mine. After a kernel upgrade, the Wifi wasn't available.

I can't find a capture of the error, but the fix is to make sure the kernel headers are installed.

uname -r
apt search linux-headers-$(uname -r)

then check for those files

ls -l /usr/src/linux-headers-$(uname -r)

If they don't exist, then install them with

apt install linux-headers-$(uname -r)

after a reboot, your wifi should be working again.