Activating a Drip Campaign can be done only when your campaign has been fully completed with no missing parts, such as an email step missing an assigned template to send.
Once your campaign is ready, you can activate it by clicking 'Activate Drip Campaign'
Activating the campaign will make it available to all of your agents. Once activated, it cannot edited. The only way to deactivate the drip campaign is to 'Archive' it, which will pull it back from all agents and halt any ongoing contact journeys. If you need to make edits after activating it, it may be best to copy the drip campaign, and archive the original, though keep in mind that contact journeys will not carry over to the new version.