When you try to activate a trigger campaign, you get an error "Smart Campaigns may not be activated with Warnings in the Smart List."
Return to the smart list for the campaign. One of the filters or triggers will have a read squiggly line under it. Hover over the red squiggle and your error message will appear.
Correct the error in the trigger or filter and refresh the page to make sure your changes have resolved the issue. Once the error is gone, you should be able to activate the campaign.