The periodic table is not complete, it is just the elements we have found, or made. There is nothing preventing us from adding another row to the table if we make more elements. In fact we have done this twice already which is why there is the strange insert into the table.

The problem is that elements with higher atomic numbers then uranium is not found in nature because they are highly radioactive. Any elements formed in supernova events will have deteriorated by the time it formed into planets. This is as I recall because the optimal number of gluons for the mixture of protons and neutrons is not a whole number and therefore never achievable.

But if you pay attention this does not mean that there is a second area of stable elements further down in the periodic table of elements. In fact there might be stable isotopes of some of the last elements we have made. We have not found any of them on Earth, or by looking elsewhere in the universe. However this does not mean such an element does not exist. It is possible that in the optimal conditions in a supernova some of these elements might be formed. And these might then become part of a globular cluster and form ore vains in planets in these. It is highly unlikely but still theoretically possible.

What is more likely is that we might find planets rich in elements that we desire. It can be as simple as iron, coal, silica or aluminium, but might also be nickel, lithium or chrome. In literature it is not important exactly which element it is so they just call in unobtainium partly as a placeholder and partly as an in-joke.