distinguish between attainable and non-returnable combinations …

An attainable combination of a set of two goods is as follows:

It is resulted from resource allocation decisions which are made by deciding what to produce and in what quantity. Its the decisions regarding production of one good vs the production of another good.

By making more one type of good (for instance chocolate bars), production of another type of good (for instance toffees) will suffer.

It also describes scenarios where the production is inefficient. That one person could be making 10 chocolates and 5 toffees, but instead you only make 5 chocolates and 2 toffees (inefficient combination)

NON-attainable: This is the production of one good vs another which cannot be done. Because your productive capacity is limited due to the limited resources.

You want to make 20 chocolates and 15 toffees but you cannot due to lack of resources. So therefore, they are unattainable. So that shows relationship of opportunity cost and scarcity.

You may increase productive capacity, and shift your production possibility curve(which shows the two combination of goods), and that way you may achieve something unattainable in the short run.