One of the most common rules for writing I see being thrown around is the fact that there are no rules. You are able, in theory, to do anything with what you write. You can kill off characters, create a character from a non sentient object such as a pebble, or set your entire plot inside the mind of the protagonist.
The problem with this rule is that there are in fact rules that you should follow when writing. You need to watch your spelling and grammar, you need to make sure that it has characters, a setting, and an exciting plot, and you need to make sure that its not disrespectful to any group of people. However there are exceptions for all these rules except for the last one I named.
One of the things I like about the novel “Flowers for Algernon” was the fact that it began and ended with the worse spelling and grammar you can possibly have without it becoming difficult to understand. This was because the novel is about a mentally disabled adult going through a medical experiment, and it explores the things that make us human and/or normal. The choice to start and end the book with bad spelling and grammar made it seem like the protagonist (the subject of the experiment) was writing down everything he was going through as his IQ increased and he was going through some other changes.
So yes there are rules. You should follow them, but in the end all rules actually are meant to be broken (except for a couple which I listed above and which don’t include spelling or grammar). However don’t break rules just because you are too lazy to go through your work and revise it. There is a time and place for everything and this includes breaking rules.