Bash script error: "function: not found". Why would this appear?

I'm trying to run a bash script on my Ubuntu machine and it is giving me an error:

function not found

To test, I created the following script which works fine on my laptop but not on my Desktop. Any ideas as to why? My laptop is a mac if that's relevant.

#!/bin/bash

function sayIt {   
   echo "hello world"
}

sayIt

This returns "hello world" on my laptop, but on my Desktop it returns:

run.sh: 3: function not found hello world run.sh: 5: Syntax error: "}" unexpected