If you're sending emails in a sequence and want to give recipients the option to unsubscribe, you can quickly enable this setting across all emails in that sequence. Here's how to do it:
Go to the Sequences tab on your sidebar, then click View Sequence on the one you'd like to update.
Once you're in the sequence, look just below the title and select the Settings tab.
Scroll to the bottom of the settings page and find the setting labeled "Include unsubscribe link in all emails."
Check that box, hit Save, and you're all set!