x is your input of the function, and within the function body, you overwrite it by assignment. and you don't have any output in this function. after executing, x goes out of scope.
try this if you really want a separate function to do this (although I don't understand why you are so keen on this lol):
function y = createZeroColumn(n)
y = zeros(n, 1);
Use y = createZerosColumn(56); to call this function, and y is the result you want.