chrome.storage.sync to save checkbox state --> error:"Uncaught TypeError: Cannot read property 'sync' of undefined"

Question

I am struggling to save the state of a checkbox with the chrome.storage.sync The Checkboxe should keep being checked/unchecked after page refresh. My current code looks like this:

<script>
function save(){
    var checkbox = document.getElementById('option1');
    chrome.storage.sync.set('option1', checkbox.checked);
}

function load(){    
    var checked = JSON.parse( chrome.storage.sync.get('option1'));
    document.getElementById("option1").checked = checked;
}

load();

</script>

its not saving the state of the checkboxes and I always get the error:"Uncaught TypeError: Cannot read property 'sync' of undefined" even if I added the "storage" permission to my manifest.json

really need your help guys :D every suggestion is highly appreciated!