Spelling, grammar, and punctuation is everything. Too many spelling errors can cause people to get pulled out of the story, and if they're really bad they can go as far as confusing your reader.

Without naming names, I read a Lapis x Reader that I thought would be good based on the cover. When I clicked on it, there was several typos in the book description before I even started reading the book. This was a red flag for me, but I ignored it and decided to read the book anyway. I couldn't even get five sentences into it. The amount of typos, the lack of periods, and the fact there was no quotations marks made it feel like a page long run on sentence. I couldn't understand any of it.

Now don't get confused, I'm not saying that you're a bad writer for making little mistakes. Everyone makes mistakes. During the process of re-writing some of my stories, I noticed so many spelling, grammar, and punctuation mistakes I wanted to take a time machine back to 14 year old me and shake them to get it together. As a writer, mistakes are bound to happen. When you're getting all your thoughts out onto the page you aren't usually focusing on things like punctuation. This is one of the times when a proof reader is most useful, because they can see the small details you don't.

Ignoring the fact that it was a rushed story, the spelling and grammatical errors made it nearly impossible for me to read. Now I'm not saying all this to bash the author of that fan-fiction. There's no such thing as a bad author, and I'm sure this author's story was good. But the way they rushed it and didn't read it back before posting it made it so that their idea was lost in the execution of the book.

A run on sentence like this with no punctuation or periods or commas to divide it up with make your readers be confused and exhausted because there is no pause to separate your ideas but instead it just keeps going on and on as if youre asking for your readers to be mentally winded plus the lack of punctation makes it so the whole thing just looks like words thrown up onto a page.

That's a lot for your reader to have to process. Since there is no punctation, the reader needs to take the time to dissect the word jumble and figure out what it is you're trying to say. After awhile that can become very exhausting, and make the reader no longer want to read your book. Always make sure, no matter how boring it is to read your own writing, to read a chapter back and make sure it is exactly the way you want it to be before posting it. You never know, maybe when you read it back for spelling errors, you'll find something you want to change that you didn't notice before.