Solution

All of your code seems fine. but the tiny error you have is, you forgot the () while defining your function. here's the code. you can copy, and paste this:

var prompt = document.getElementById('prompt');
var a = document.getElementById('a');
var b = document.getElementById('b');

prompt.innerHTML = "Hi! I'm Walt, your programming language adviser. Before we begin, have you learned a programming language before?";
a.innerHTML = "Yes!";
b.innerHTML = "No, i'm totally new!";
a.onclick = "a1";
b.onclick = "a2";

function a1() { // adding ()
	prompt.innerHTML = "That's great! What one is it?";
	a.innerHTML = "Python";
	b.innerHTML = "Java";
	var c = document.createElement("button");
	c.innerHTML = "JavaScript";
};

so that way you are defining it properly. for every function it is like that.
That should work

Also, just saying, prompt is a reserved function for getting user input. You won't have any issues with it in the short term, but you should definitely rename it.

Secondly,

you never actually added this element into the document, so you won't ever see it.

Error

functions must have an argument list, even if it may be empty

Solution

function a1() {

so easy!

You put function a1 {} instead of function ai() {}

uhh for html maybe just do:

<p id="prompt">Hi! I'm Walt, your programming language adviser. Before we begin, have you learned a programming language before?</p>
<button id="a" onclick="a1">Yes!</button>
<button id="b" onclick="b1">No, I'm totally new!</button>

inside a1 js:

document.getElementById('prompt').innerHTML="That's great! What one is it?";
document.getElementById('a').innerHTML="Python";
document.getElementById('b').innerHTML="Java";
document.createElement("button").id="c';
document.getElementById('c').innerHTML="Javascript";
document.createElement("button").id="d';
document.getElementById('d').innerHTML="C++";

Try that.

You need () after your function name