Iron can't be fused in stars to produce any energy because the fusion of iron nuclei requires energy input instead of energy release, which is unlike the fusion of lighter elements.
The energy absorption causes the process of fusion to halt and makes the end of the star's life as a fusion reactor.
Nuclear fusion is basically the process of combing of lighter atomic nuclei into heavier ones and releases energy in the process.
The energy release occurs as a result of the resulting heavier nucleus which has a slightly lower mass than the combined mass of the original nuclei, due to the conversion of mass into energy, (E=mc2".
And for elements that are lighter than iron, the energy release is a natural consequence of the process of fusion.
Iron and most specifically iron-56 also has the highest binding energy per nucleon of all of the elements which means that it's nucleus is exceptionally stable and also tightly bound.
And when the iron nuclei are fused, the resulting nucleus would also have a slightly higher mass than the combined mass of the original iron nucleus and the fusing particle like helium nucleus and so instead of releasing energy, the iron fusion requires an input of energy to occur.
And as a star ages, the star then fuses lighter elements into heavier ones and eventually produces iron in it's core.
Then after the core is primarily composed of iron, the process of fusion can no longer sustain itself because it would start absorbing energy instead of releasing energy.
This then leads to a dramatic decrease in core temperature and pressure and causes the star to collapse under it's own gravity.
And the collapse can then result in a supernova explosion, which is a spectacular event that can create heavier elements through other nuclear processes such as neutron capture.