Land, labor, capital and entrepreneurship are the resources that are used to make products. Those four categories of resources are limited, regardless of the amount of money in the marketplace. Lots of money and limited products just lead to higher prices. Prices go up, but quality and quantity of products does not. Giving everyone lots of money doesn't solve the problem.
All economies have limited resources no matter how rich a country is.
Because of this problem of scarce resources, people need to make the most of what they have by making choices that best allocate those limited resources of land, labor, capital and entrepreneurship. This problem of scarcity forces us to make decisions. Whenever we make a choice, we gain something and give up something. When you choose one item, you give up the opportunity to have the other item. The alternative item becomes the opportunity cost.