When setting up a Broadcast on a Journey, you'll be asked to fill in a few things: the email you want to send, the subject line, and campaign tags.
Campaign tags can be easy to miss, but they're required. If you skip them, or don't save them correctly, you'll see a red "Please supply Campaign Tags" error at the top of the page when you click Create.
To avoid this error, find the Tag Content section near the bottom of the setup page.
Type a tag into the text box, or choose one from the dropdown.
Then click Add Tag Word to save it.
You can add as many tags as you like, one at a time. To remove a tag, click the X next to it.