Quote - A simple question, is the speed of light the same in any direction measured on the surface of the Earth ? If yes then Sagnac effect does not exist, if no then Relativity doesn't exist.

Wrong! As I said earlier (and you ignored) a positive result using special relativity was predicted in 1911, two years before Sagnac made his experiment. So the Sagnac effect don't really contradict relativity, even if Sagnac himself thought so.

Quote - Anyway the Theory of Relativity has very little importance because has a very small use and in electronics has no use at all. You don't need Realtivity for television, cell phones, computers, comunications, robots, etc, etc, etc

GPS. And relativity is important in communications using satellites due to the fact that satellites are travelling at a significant fraction of the speed of light.

The design of cathodic tubes for TV must take in account relativistic mass increase due to the electrons travelling at a very considerable speed. The same is true for the design of free-electron lasers.There are probably many others that I don't know just now.

And there is something that you should know about electronics. You have to take in account quantum mechanics to design computer circuits, right? Then, you should know that the Dirac equation, one of the fundamental equations of quantum mechanics, incorporates relativity.