Deuteronomy 12:32 Whatever I command you, be careful to observe it; you shall not add to it nor take away from it.
When Moses was reminding the people of Israel about the commandments God had given them it was not just the Ten Commandments but the whole list of them.
God was trying to get the people to turn away from their evil ways by sticking to the laws as best they could. When they couldn’t He gave them forfeits or sacrifices they had to do.
God does not want us to make up our own rules because we will make them so we can live more easily with them. God wants them to be right and that means sticking with them as given by Him.