A new registration or DNS change is not visible everywhere at once. Registries, nameservers, and recursive DNS caches need to return the updated information.
Check that the domain is registered, its nameservers are correct, and the required DNS records exist at the provider hosting the DNS zone. If the domain is meant to open a SmallHost website, also confirm that Plesk contains the domain and that its certificate covers the name.
Avoid making repeated DNS changes while testing. Each new change makes it harder to tell which version a resolver is using.