I cannot get to the next chapter since I don't have enough stars.
Even though it is possible to speed run through the game you will need a certain amount of stars in order to progress to new chapters. High-level boxers will simply refuse to fight against contenders with low recognition.
If you find yourself in the situation in which you will be refused to progress then simply go back to the map and fight some of the fighters from the earlier map stages. They should not be a match for you anymore but they will give you a desired star, coins and experience points which you can later spend on your boxer statistics and items.