Well there are several things wrong with this code.
- You can't use size() to extract things from an array. Replace frq= size(Signal,1); with frq = Signal(:,1);
- You can't use an array as an index (this is the cause of the error message since Signal will be an array after reading the data from your spreadsheet). Use, for example, for i = 1:numel(frq).
Also, though not wrong, there is an easier way to find your 'x' without using a loop: x = S12(frq==2.45)
If you are planning to use this in a Simulink Matlab function block (I'm just guessing based on the coder.extrinsic call), I would recommend against repeatedly calling xlsread in a Simulink simulation unless, for some strange reason, you expect text.xlsx to be changing during the simulation run.
There is probably a better way to achieve what you are trying to do. If you explain what you are trying to do you may get some suggestions on how to do it more effectively.