First a couple of things about your existing code.
1. You are assigning different default values to the $fname story variable and the related fname input field, this will cause an issue if the Reader decides not to edit the Player value and clicks the Confirm link without clicking on the Save button first.
You should assign the same default value to both the story variable and the input field.
Please enter your player name:
(set: $fname to "Player")
<input type="text" name="fname" value="Player">
<button type="submit" onclick="CustomScripts.updateVariable('fname')">Save</button>
2. You don't need to load jquery-3.3.1.min.js within your Submit passage.
Harlowe already contains a copy of jQuery so you don't need to load another copy, and even if you did need to load a different version of jQuery the URL you are using won't work because there isn't a copy of the jquery-3.3.1.min.js file within the same location as your story HTML file.
I suggest removing the following from your Submit passage.
<script src="jquery-3.3.1.min.js">
</script>
3. Using wrong (and undefined) story variable name in Submit passage.
The Player's name is stored within the $fname story variable, but you're incorrectly trying to use harlowe.State.variables['inputName'] to access that value in your sendData related JavaScript.
Change your sendData related code to the following..
var sendData = JSON.stringify({
"inputName" : harlowe.State.variables['fname'],
"history" : harlowe.State.variables['history'],
"now" : Date.now()
});
I suggest making the above changes and testing again.