$CIRCLE_TAG is only present during tag builds (i.e. it won’t show when building branches).
Just as a note: if you’re on the CircleCI dashboard and you’ve clicked on a branch to view all of its jobs, you won’t see the tag building (since a tag is not a branch). Say your repository is called test-repo and your branch that you have tagged is develop. If you’re clicked on the develop branch on CCI to view all the branches jobs you’ll have to click on test-repo to see all jobs for that repo - it’s the only way you can view the tags being built, from what I can see.