Why are elements with atomic number 26 not formed by nuclear fusion in the cores of very massive stars?

You started and ended with four nucleons (protons+neutrons=4), for nucleons—protons plus neutrons—must be conserved in the fusion reaction. However, the helium nucleus has less energy per nucleon than the protons you started with. This makes the fusion reaction energetically favored when you fuse four protons to create a helium-4 nucleus (and also absorb two electrons for charge balance).

Fusion must conserve total nucleons (= protons + neutrons), but as energy per nucleon drops, you can still release energy. Continue fusing neutrinos and increasing in atomic number. As you pass through carbon (atomic number 6), oxygen (8), silicon (atomic number 14), etc., the energy per nucleon keeps decreasing.

However, the problem arises at iron (atomic number 26), where the energy per nucleon reaches a minimum, then stops decreasing and increases again; this means that energy cannot be released by fusing iron nuclei any more. The abundance of elements rapidly decreases beyond iron, and the only ways to produce heavier nuclei are through processes like neutron capture or supernova explosions (see https://en.wikipedia.org/wiki/Nucleosynthesis).

The fusion process in these stars can produce elements up to iron through successive fusion reactions; beyond iron, the fusion of lighter elements consumes more energy than it releases, making it energetically unfavorable for stars to produce heavier elements through fusion. Therefore, elements with atomic number 26, such as iron, are not formed by nuclear fusion in the cores of very massive stars.