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

Chances are that on your desktop you are not actually running under bash but rather dash or some other POSIX-compliant shell that does not recognize the function keyword. The function keyword is a bashism, a bash extension. POSIX syntax does not use function and mandates the use of parenthesis.

$ more a.sh
#!/bin/sh

function sayIt {   
   echo "hello world"
}

sayIt
$ bash a.sh
hello world
$ dash a.sh
a.sh: 3: function: not found
hello world
a.sh: 5: Syntax error: "}" unexpected

The POSIX-syntax works in both:

$ more b.sh
#!/bin/sh

sayIt () {   
   echo "hello world"
}

sayIt
$ bash b.sh
hello world
$ dash b.sh
hello world