Create a new page
Create a new node of type Basic page
Use below sample code to create this page.
<h1>Error 403 - Access denied</h1>
We are very sorry, but you do not have access to this page.
Set this page as default Error 403 page
When creating page, make sure you select Full HTML as Text format.
You have now successfully created Error 403 (Access denied) page.
Navigate to: Administration >> Configuration >> System
And set this page as your Error 403 page and click Save Configuration button.