Why are elements with atomic number 26 not formed by nuclear fusion in the cores of very massive stars?
Elements with atomic number greater than 26 are not so formed because it is not energetically favorable.
When you fuse four protons and to make a helium-4 nucleus (and also absorb two electrons for charge balance), it's energetically favored because the product nucleus has less energy than what you put in. You started and ended with four nucleons (protons+neutrons=4), for nucleons -- protons plus neutrons -- must be conserved in the fusion reaction. But the helium nucleus has less energy per nucleon than the protons you started with.
Keep fusing neclei and going up in atomic number, and the energy per nucleon keeps going down as you pass through carbon (atomic number 6), oxygen (8), silicon (14), etc. Fusion must conserve total nucleons (= protons+neutrons), but as energy per nucleon decreases you can still release energy.
But at iron (atomic number 26) you have a problem. The energy per nucleon hits a minimum, stops decreasing and goes up again. You can't release energy anymore by fusing iron nuclei. There are other mechanisms for making heavier nuclei such as neutron capture or supernova explosion processes (see https://en.wikipedia.org/wiki/Nucleosynthesis), but these are harder to come by than stellar-core fusion. The abundance of elements beyond iron rapidly falls off.