It is an unfortunate aspect of Matlab that the syntax for functions and variables is so interchangeable, to the extent that two things that should be clearly different not only use identical syntax, but the definition of one hides the other.
If 'signal' is neither a variable nor a function in your path then that is the error message you will get, but it is impossible for us to tell whether your intention was that 'signal' be a vector/array variable being indexed or a function with y as its argument.
