Disable Submit Until Checkboxes are Check

I am trying to enable a submit button if a user checks one of four check boxes.

Here is my Java:

<script>
$(function() {
    $('input.required').click(function() {
        var unchecked = $('input.required:not(:checked)').length;
        if (unchecked == 0) {
            $('#submitBtn').removeAttr('disabled');
        }
        else {
            $('#submitBtn').attr('disabled', 'disabled');
        }
    });
});
</script>

Here are my form elements:

            <input class="required" name="equipment_dropped_off" type="checkbox" id="check1" value="equipment_dropped_off" onclick = "document.getElementById('submit').disabled=false;"/>
            <label for="check1"><span class="style1">Equipment Dropped Off &nbsp; &nbsp; </span></label>
            <span class="style1">
            <input class="required" name="work" type="checkbox" id="check2" value="work"/>
            <label for="check2">Work performed &nbsp; &nbsp; </label>
            <input class="required" name="payment" id="check3" type="checkbox" value="payment" />
            <label for="check3">Payment Recieved &nbsp; &nbsp; </label>
            <input class="required" name="equipment_picked_up" id="check4" type="checkbox" value="equipment_picked_up" />
            <label for="check4">Equipment Picked Up</label>

    <input class='btn-style' data-rol="none" id="submitBtn" type="submit" name="submitBtn" value="STEP 4 - SAVE SIGNATURE" disabled="disabled">

As soon as I check a box, my browser console shows: Cannot set property 'disabled' of null

What am I doing wrong?