Summary:
Given a Git repository which has already been discovered, but still has no Jenkinsfile, it's possible to go from the Classic UI to Blue Ocean. When doing so, it appears that Blue Ocean will default to the "Branches" tab, instead of the "Activity" tab. This is a problem, because in the Branches tab, there is no "Create Pipeline" button presented.

It was jarring, because I thought something was broken. "Where'd the Create Pipeline button go?"

Steps to recreate:
1. Using the Blue Ocean Create flow, configure an existing repository on a plain Git server (although I doubt the SCM in use actually matters).

2. Exit to Classic View, and navigate to the multibranch project you just created if you're not already there.

3. Click Open Blue Ocean on the left hand side, and you'll be taken to Blue Ocean. Notice that we're on the "Branches" tab, and the only button available is "Learn More:"

4. Click on the "Activity" tab, and the "Create Pipeline" button becomes visible:

5. As an aside, the "Pull Requests" tab is also available for clicking, and you get a (perfectly true) message stating that here are no open pull requests. But there are also zero buttons to click on now - neither "Create Pipeline" nor "Learn More" are shown: