Apps Script Refused to Connect

You need to set the option XFrameOptionsMode to ALLOWALL.

XFrameOptionsMode

https://developers.google.com/apps-script/reference/html/x-frame-options-mode

Setting XFrameOptionsMode.ALLOWALL will let any site iframe the page, so the developer should implement their own protection against clickjacking.

return template.evaluate().setSandboxMode(HtmlService.SandboxMode.IFRAME)
.setXFrameOptionsMode(HtmlService.XFrameOptionsMode.ALLOWALL);

As stated in the comments below (Shan Eapen Koshy said) check that your browser is logged into one Google account only.