400 Error POST Method to DynamoDB - AJAX - json

Question

I am currently trying to update a DynamoDB table with information that I add into a HTML form. Every time that I run my program and submit my information, I get this error:
POST https://quovje1gi8.execute-api.us-east-2.amazonaws.com/production/accounts 400 ()
I can run my Lambda function on AWS, and add data to my table just fine. I have checked again and again to make sure that my permissions are established correctly and they are. I just can't seem to figure this out.
My front-end:
<form>
<label for="username">Add Username:</label>
<textarea id="username"></textarea>
<button id='submitButton'>Submit</button>
</form>
<script
src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js">
</script>
<script type="text/javascript">
var API_URL = 'https://quovje1gi8.execute-api.us-east-2.amazonaws.com/production/accounts';
$(document).ready(function(){
$.ajax({
type:'GET',
url: API_URL,
success: function(data){
$('#accounts').html('');
data.Items.forEach(function(accountsItem){
$('#accounts').append('<p>' + accountsItem.username + '</p>');
})
}
});
});
$('#submitButton').on('click', function(){
$.ajax({
type:'POST',
url: API_URL,
data: JSON.stringify({"username": $('#username').val()}),
contentType: "application/json",
success: function(data){
location.reload();
}
});
return false;
});
Lambda function:
'use strict';
console.log('Starting Function');
const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient({region:'us-east-2'});
exports.handler = function(e, ctx, callback) {
var params = {
Item: {
user_id: ,
date: Date.now(),
username: "",
password: "",
name: "",
location: "",
field: "",
company: ""
},
TableName: 'accounts'
};
docClient.put(params, function(err, data){
if(err){
callback(err, null);
}
else{
callback(null, data);
}
});
}