Hi there,
This error page comes from GitHub, not Netlify, so one presumes your DNS points to GitHub for your custom domain still. Please check out these articles that describe the correct configuration and what might be preventing a reconfiguration from taking an effect:
- [Support Guide] DNS Quick Start - How to set up DNS (how to configure)
- why your changes may take Some Time to take effect: [Support Guide] Why do DNS / SSL changes take up to 48 hours to propagate? (TTL)
If after things have propagated - you should be able to see your changes on a website like https://whatsmydns.net showing identical details to what shown in that first article - things still aren’t working as you expect, please let us know your hostname so we can examine it directly and advise. The fact that you didn’t tell us to start with has ensured that our answer can only be in general, not specific terms thus far.
But since I’ve now equipped you to debug yourself, let us know how it goes 