Iron fusion cannot support a star because

Because it consumes more energy than what it produces

Explanation:

Nuclear fusion is the process that produces energy inside the core of the stars. In nuclear fusion, two lighter elements fuse together to form a heavier nucleus: the mass of the final product is smaller than the total mass of the initial nuclei, therefore the mass lost in the process is converted into energy, according to Einstein's equation

where c is the speed of light.

The initial nuclear fusion process in the stars is the fusion of hydrogen nuclei into helium. When the star runs out of hydrogen, the fusion of the helium nuclei starts, and so on.. when the star runs out of one element, the fusion of the next heavier elements starts.

This process continues only until the star reach the iron fusion. However, iron fusion does not occur. This is because the final mass of the product of the iron fusion is larger than the initial mass of the iron nuclei: this means that this process does not produce energy, rather it consumes energy. So, the nuclear fusion in the star stops, and the star will eventually die.

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