Here are some important point to address while making On page. - Content and Programming score ratio. - Over wheeled use of keywords on page. - Don't use multiple time h1,h2 tags - Outbound links to irrelevant website. - Avoid issues Do technical audit of your website. These are quite simple stuff to consider.
Keyword Stuffing or No keywords at all, Low-quality/plagiarized content, not paying attention to the responsiveness of a website, ignoring tools liek SEMrush or Google speed insights, not paying attention to Analytics at all.
I think keyword cannabilism- careful you don't use the same focus keyword across too many pages. It can be hard especially in ecommerce where you have subcategories etc