RESOLVED: This issue was resolved by the Ensighten Product team.
When creating a new tag or merging tags, the section Choose where and when the tag should fire shows a blank default condition.
What to Do
You can work around this issue by changing to a temporary space, then choose the space you want to use. This action refreshes the condition list and places All Pages - Non-blocking (Asynchronous) as the default condition.
What is Happening
The Ensighten product team is actively working on this issue.