The message "Custom domains cannot be used on public applications" indicates that your app permissions are currently set to 'Everyone can edit.' To resolve this issue and change your domain, you need to adjust your app's settings to private. Here's how you can do that:
Go to your app editor.
Navigate to the Settings tab.
Select the General section.
Change the application rights of your app to a private app.
After making these changes, you should be able to edit your domain settings as needed. If your app is already set to private and you're still encountering this issue, please let us know and we'd be happy to take a closer look.
