Hi @Robert94
You can use the Design studio from power pages to show the Access denied page.
Here are the steps:
1. log in to https://make.powerpages.microsoft.com/
2. Choose the right environment
3. Click Edit > then the site will be opened in the new Design Studio

Hope it helps.
------------
If you like this post, give it a Thumbs up. Where it solved your request, Mark it as a Solution to enable other users to find it.