Final answer:
In an Email Performance report, the column that you cannot add is 'Engagement score'. The report typically includes metrics such as Hard Bounced, Unsubscribed, and Clicked, but not an overall 'Engagement score'.
Step-by-step explanation:
The column that you cannot add in an Email Performance report is Engagement score. The options typically available in an Email Performance report include metrics such as Hard Bounced, Unsubscribed, and Clicked. These metrics are designed to track the actions that recipients of the emails have taken, such as whether the email could not be delivered (Hard Bounced), the recipient has chosen to no longer receive emails (Unsubscribed), or the recipient interacted with the email content (Clicked). However, 'Engagement score' is not a standard column used in email reporting; this term might be used more broadly to assess the overall engagement a subscriber has with a series of emails or campaigns, but it is not a standard measurable metric in individual performance reports.