I assume this is the website example you refer to? It works for me when I create and run the problem in a collection. Have you created your problem in a course?
In courses hosted on grader.mathworks.com, courses initially only have access to base MATLAB. In order to use any of the toolboxes, they must first be added to the course. See the Select Products for Courses page for instructions on how to do this. Once you add the Signal Processing toolbox, you should be able to run the code without an error.
As an aside, collections have access to all toolboxes. You can use a collection to create a repository of problems that you can then draw on it to build a course. You'll stil have to add the products to the course for students to solve the problems, but it can make the creation process a little easier. For more on this, see Ch 6.2 of Teaching with MATLAB.
