customising error message

Hi there!

Thanks for your query.

This 403 error message comes from the Joomla core. There is no default way to modify it as per my knowledge.

You can only customize the 404 page. Helix ultimate has the option to customize it within the Basic tab of the Template Options:

https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/basic#error-page

Or, you can create a custom 404 error page either. Here is a blog about it:

https://www.joomshaper.com/blog/create-a-custom-joomla-404-page-with-sp-page-builder

Best Regards