Once you create your unsubscribe pages, you can validate them in different statuses:
No Unsubscribe Template Selected
If you add a custom unsubscribe link to your email design but do not select an unsubscribe page, you will see this notification on the launch step.
In this case, you need to go back to your email design, select the Unsubscribe block as the template, and select the custom unsubscribe page you want to use in this particular email campaign.

No Unsubscribe Link Added
You will see the following notification if you haven’t added unsubscribe links to your email design.

If you haven’t added any unsubscribe links to your email design, you will see the following notification: You cannot launch your campaign unless you add an unsubscribe link. In this case, you need to go back to the design step and add your global unsubscribe or preference center link.
Default Unsubscribe Link
If you proceed to launch your campaign, the system will automatically add the default unsubscribe link to your email.

