Your code does not satisfy the requirements of the assignment. The problem statement says:
Create a function file that accepts a vector of any length and return a vector whose elements are arranged in descending order.
Your function displays the sorted vector and returns the value of the temporary variable the for loop uses to iterate over the array.
I would omit the disp call because what if the user of your code calls it inside a loop that iterates a thousand or a million times? Do they expect or want your code to display a thousand or a million vectors? Probably not.
Right now your code operates on the vector a that it received as input. That's fine to do inside your function (generally speaking, whatever you do in your function's workspace doesn't affect anything outside its workspace until you return something from your function.) Since your assignment constrains what you're allowed to call the output argument you probably want to assign the vector on which your code operated to the required variable name at the end, just before the function returns.
If this weren't an assignment I'd probably just use the same variable name as the input and output arguments.