If you can't track your campaign, it's because we can't find the YAMM ID notes anymore. Please consult this article to fix this issue. If you forgot to activate the tracker, it is sadly too late and impossible to track your campaign.
Why do I get this error?
When checking your read receipts, if you see the error 'Sorry, it seems tracking hasn't been activated for this campaign', it means we didn't find the read receipts IDs in your spreadsheet.
If you have checked the box to track emails opened when sending emails, we create a unique identifier for each email sent, and store it as a comment / note on the cells of the 'Email Address' column - the YAMMID.
When you open the sidebar to check for read receipts, we try to get those unique identifiers (notes in your sheet) to check in our database if those emails have been opened.
If you can't see those YAMMID as notes / comments in your sheet, it means email tracking hasn't been activated for your merge. If you forgot to enable the tracker, you will not be able to track your campaign.
But maybe you deleted those notes, or accidentally clicked on 'Follow-up' from the tracking report: In these last two cases, you can use the revision history of your Google Sheet to restore those notes and fetch your receipts.