The error "You forgot to include an unsubscribe link" can be displayed when saving your emails if there is no standard Footer block or {$unsubscribe} variable added to the email content. Resolving it depends on the campaign editor you used.
Drag & drop editor
a. Use a standard Footer block
If your campaign was created using the Drag & drop editor, simply add a standard Footer block (such as the Centered footer or Basic footer), then save your changes.
All standard Footer blocks (e.g., Centered footer, Basic footer) within the Drag & drop editor have the required {$unsubscribe} variable hardcoded into them internally.
b. Use a custom unsubscribe link (no standard Footer block)
If you prefer to add an unsubscribe link to your email content manually and do not want to use a standard Footer block within the Drag & drop editor, the {$unsubscribe} variable must be added as a hyperlink to the custom unsubscribe text.
How to link it: Highlight your custom unsubscribe text, click the Link icon on the inline toolbar above the text, select Insert Variable, and choose Unsubscribe.
Custom HTML editor
If your campaign was created using the Custom HTML editor, make sure the {$unsubscribe} variable or the complete code snippet below has been added to your campaign's HTML content:
<a href="{$unsubscribe}">Unsubscribe</a>