I think your code is working pretty much as you might expect it to. The only obvious mistake is that you are setting radioInterface.type = "submit";
where you presumably mean buttonSubmit.type = "submit";
.
It should be apparent that this works fine if you actually put some content in to make elements like label
actually visible...
Something like this, perhaps?
var form = document.createElement("form");
var fieldset = document.createElement("fieldset");
var legend = document.createElement("legend");
var labelFunction = document.createElement("label");
labelFunction.textContent = 'Function'; // <-- added
var labelInterface = document.createElement("label");
labelInterface.textContent = 'Interface'; // <-- added
var labelComment = document.createElement("label");
labelComment.textContent = 'Comment'; // <-- added
var radioFunction = document.createElement("input");
radioFunction.type = "radio";
var radioInterface = document.createElement("input");
radioInterface.type = "radio";
var textarea = document.createElement("textarea");
var buttonSubmit = document.createElement("input");
buttonSubmit.type = "submit"; // <-- corrected
I believe your code to actually add them to the document should be working fine.