What error is there

@vaibhavambatkar14 In line 11, you have already closed the tag . You have repeated the closing tag in line 12 with no opening tag before it.
Also, tag is repeated again in line 5. You dont need it if you have already mentioned it in line 4.

@vaibhavambatkar14
Make sure that you have closed the tag in line 4.
Remove the and the tag in line 11.

@vaibhavambatkar14

Can you please share the complete code?

@vaibhavambatkar14,
Here is the right code,

<html><head>
    <title>Bolt IoT Platform</title>
    <script type="text/javascript" src="https://cloud.boltiot.com/static/js/boltCommands.js"></script>
    <script>
    setKey('{{ApiKey}}','{{Name}}');
    </script>
</head>
<body>
    <center>
    <button onclick="digitalWrite(0, 'HIGH');">ON</button>
    <button onclick="digitalWrite(0, 'LOW');">OFF</button>
    </center>
</body></html>

In your code ,line 4 it’s required to adding a space before src attribute.
And line 11 you close body and center tag that’s why it showing error at 12 & 13 line.