If you do not run that command before /boot fills up, and you have a full /boot with a partially installed kernel, then that command fails. So this works fine if you remember to call it regularly, but it does not solve the problem once it occurs.

Interesting. I haven't encountered that edge case. I've many times filled /boot and resolved by doing an auto remove.

It seems silly to me that I need to manage this myself. Why do I need to be worrying about different kernel versions? I just want to make websites.

Following the chain of links and answers and explanations, we come to the conf file that says in the comments that it commonly results in two (2) kernels being saved, but can sometimes results in three (3) being saved.

IOW, it does automatically remove old kernels, it just keeps the last 2-3.

So, yes, run "apt-get autoremove", that's it.

I think it has solved the problem for me, but still is not a good solution for anyone who would answer "What's a terminal?"

I love having a terminal with bash and use it constantly, but I don't think it should be needed for the system to just go on working.