This error occurs when the "Subscription tracking" setting in the Sendgrid is turned off.
It can be resolved by turning on the setting mentioned above.
Please follow these steps
1) Login to the Sendgrid https://app.sendgrid.com.
2) Select the Business you would like to view.
3) Tap on Settings and select Tracking.
4) Turn on the "Subscription tracking" setting.
Once, you turn on the settings, the Subscription tracking becomes active, and are able to add the subscribe link to the campaign (Email template).