Can you post screenshots please?

The same problem is happening when tray to add ad sense code to plugin

<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-116836615-1"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-116836615-1');
</script>

If second screenshot is following immediately the first screenshot, talk to your webhost. They have a shitty security filters that blocks you because they think your analytics code is a hack.

This "Forbidden" page is NOT generated by b2evolution.