Everyone who does their own SEO makes at least one mistake at first. Hopefully, it's not leaving the site on no index for months, but we can all learn from other's mistakes. Most mistakes come from reading outdated information online from supposed experts who give advice that worked in the beginning of their career but it's outdated now.
Buying backlinks is a huge mistake. You don't get backlinks from high-quality, authoritative from someone on Fivver. Buying backlinks is cheating in Google's eyes and they will either not index your backlinks, not give them any weight or penalize your website.
Not having a Professional Design Your Website
Websites are expensive. It's tempting to let a family member make one for you, but unless they are an SEO expert, you'll get a pretty website which search engines ignore. Common mistakes include putting important text in images, slow load times and forgetting alt text descriptions for all images.
Not Doing Keyword Research
Using keywords which you believe people will use to search for your site is a good start, but you still have to do keyword research. Find long-tail keyword phrases with less competition to focus on while your site moves up in the SERPs for high competition keywords. You also have to account for regional differences in how people search if you're targeting a national audience.
Not having high-quality content is a big mistake. You site won't rank well and the visitors you get will quickly leave. Your content has to be better than the content of the websites which hold the three top organic positions.
Most newbie mistakes come from trying to save time and money. You will not land one of the top three positions on page one of Google's results overnight so forget the bad advice you've heard about ranking well quickly. Invest the time and money you need to and you'll reap long-term results. For more information click here https://www.reddit.com/r/SEO/comments/cane85/whatisyournumber1seomistake/.