If you set a breakpoint just before that line you will notice the values of all your variables: s is a scalar and L has a value of 100. Then you try to do s(L-1), which will attempt to access the 99th position in s, which doesn't exist.
You should really try using the debugging tools, starting with the warnings mlint is giving you. Make sure to resolve those. If you are absolutely sure they don't apply to your specific situation you can right-click the orange underlined code and select the option to suppress the warning on that line. None of the warnings mlint is currently giving you should be ignored.
Matlab has very flexible debugging tools including breakpoints and the option to halt execution when an error has been thrown. Use them to your advantage.