Thank you, sir for your kind attention to my issues. I tried other alternatives, like: AccuracyGoal-> Infinity and WorkingPrecision->MachinePrecision. Nothing seems to work. If I start things off with elctrn[eV_]:= I do indeed get a numerical answer. {eV -> 4.476 x 10^-6} free of error messages. Unfortunately, the correct answer (arrived at through trial and error) is 399.8 electron volts. Am I using the wrong version of Scientific Notation for my numbers? The equation in question is a popular rewrite of De Broglie's expression for particle wavelength in terms of electron volts. Not much I can do about the magnitude of the terms. I apologize to you for all the trouble I'm causing to the Mathematica community.

Trial and error. Please note. I started in with Mathematica in 2002. Never had a problem I couldn't solve by reading your tutorials. Soon as I upgraded to Windows 11 Pro in November of 2022 things went south. I suspect very few of your clients are on Windows 11 Pro. Thus, you never get complaints. Except from me.

First. I am relieved to learn you are not finding any conflict between Mathematica 13 and Windows 11 Pro. Second. A great many years ago a Nobel Laureate, Arthur Holly Compton made a careful study of the relationship between the dominant wavelength in an X Ray range energy field and the production of 'Recoil Electrons' at a given wavelength. My goal, to explain his findings in terms of the De Broglie wave / particle equations in units of electron volts. Among other wavelengths Compton reported 0.614 Angstroms (6.14 x 10 ^-9 centimeters) as especially productive of 'Recoil Electrons'. I wrote the equation (above) to search for the amount of energy in units of electron volts needed to produce non-relativistic waves at a wavelength of 0.614 angstroms. Before I tried 'FindRoot' I entered different values into the electron volt term (eV) in the equation above at the top of the page. A few minutes of labor and I arrived at 399.8 electron volts as able to produce X ray waves at 0.614 angstroms. Then I tried FindRoot. Your expression returns an unlikely value of 4.7 x 10^-6. So, next, I posted this plea for help.

To summarize, the equation you posted had elctrn[eV]==0 whereas, as Gianluca Gorni shows, you most likely intended it to be elctrn[eV]==6.14 10^-9. Which gives a solution very close to the one you had obtained by plugging in possible values.

Maybe You type a wrong expression to calculate?

It seems the only solution is x to infinity.

Your expression is always positive.

Thanks for your help and enthusiasm. Unfortunately, the correct answer is: 398.8 electron volts (I worked the answer out by trial and error). Then I thought to try 'FindRoot'.