This might be a spelling mistake, perhaps you meant to write reflect_collision.
Otherwise you will need to make sure that there is a function with that name of your path:
which reflect_collusion
If there is none found then you either need to write it, or download it, or correct its name, or add its directory to the MATLAB path, or make the function name the same as the file name. MATLAB cannot use functions that it does not know about, so it is up to you to make sure that it knows where to find all functions that it needs to run your code.
