Thank you very much. How can I fix that?? Please tell me.

Hello Sir, I have modified my code but there is an error message like FindRoot::nlnum: The function value <<1>> is not a list of numbers with dimensions {4} at {x5,x6,x7,x8} = {0.,0.,0.,0.}. >>. Please check that.

Attachments:

spectra_yac.nb

If you are a beginner, consider reading Stephen Wolfram's An Elementary Introduction to the Wolfram Language. It is available online for free here.

Actually I am beginner that's why I made so many mistakes. Thank you very much for helping me.

There are a lot of errors in the notebook.WL is case sensitive so sqrt, tanh, for and many other names need to be capitalized. Functions are defined and called with square brackets Sqrt[arg] not Sqrt(arg), f2[x1_, x2_, x3_, x4_] not f2(x1_, x2_, x3_, x4_)...

Rather than using a single cell, put each expression in its own cell, remove the trailing ; and make sure it has no syntax errors and evaluates correctly.

Most likely syntax errors. Hard to say without looking at the code. Edit your post and add the code as text (not an image).