That photo nicely shows everything that’s going on. Thank you!
So, some packages on a Debian system are packaged in a way such that you can install multiple versions of them at the same time. One example are shared libraries with different SONAME and in your screenshot you see that you still have old versions of libfmt9 or libperl5.38t64 installed. When a transition happens to a new version and as you upgrade packages depending on them, there will come the point where nothing depends on them anymore. At that point the package is still installed but its original reason for being installed is gone. If this happens to a package, apt will suggest that you can autoremove the package. This is what you see in your screenshot where apt tells you “Use ‘sudo apt autoremove’ to remove them”. Other than shared libraries, another typical package affected by this is the linux kernel itself. You can have multiple versions of the linux kernel installed at the same time. But the kernel image itself as well as the initramfs live in a partition called /boot. This partition is “only” 488 MB small, so it can only hold a certain number of copies of linux. In your screenshot one can see that apt suggests that you can autoremove six versions of the linux kernel. Why should you follow apt’s suggestion? You see one reason for doing so further down. Above the “error” marked in red you see the message “failed to write (No space left on device)”. This is telling you that dpkg failed to install a new version of the linux package because you ran out of space on your disk, likely in /boot. Fixing this should be as easy as first running:
sudo apt autoremove
This will remove a bunch of packages, most importantly the six old versions of the linux kernel which are currently crowding your /boot. And then you run:
sudo apt --fix-broken install
Which will try installing the most recent kernel image again and that should now succeed as you cleaned up old kernel images.
On Debian stable installations you usually only need to run sudo apt autoremove once every 2 years because you’ll not do considerable upgrades more often than that. All the other package upgrades in that time will be security related upgrades which typically do not lead to anything getting marked for autoremoval. But on your Pocket Reform you are running Debian unstable and from the kernels you have installed one can see that you have upgraded your system regularly. This means, that you should also run sudo apt autoremove more regularly.
You are not the first running into this. This is a related topic: /boot too small?