the main mistakes entrepreneurs make when starting out are the following (I have made them myself and I think others make them too).
Starting a project without having sufficient funds, I would recommend at least 6-12 months. This will give you security and avoid fear of losing money.
Believing that you are going to make money the next month or that the business is going to be profitable the first year.
Focusing only on money and forgetting about important things such as family and your own health (mental and physical).
They stop training themselves.
Not knowing the important metrics of their business and that leads to spending a lot of time on unnecessary things.
They do not invest in a mentor. A person who has been through everything you want will help you have clarity, focus and a broader perspective.
Those are some of the ones that come to mind right now.