Summary:
As described in JENKINS-46396, the Blue Ocean Pipeline Editor is not compatible with older versions of Bitbucket Server, in this case 4.14.4. However, a user is able to use the editor on an existing Pipeline on this old version of Bitbucket. Attempting to save these changes results in a "Precondition required" error, which isn't a very clear explanation.

Prerequisites:
A Bitbucket server running an old version of Bitbucket. In my case I'm running one at 4.14.4.

Steps to recreate:
1. Using the classic UI, create a connection to your Bitbucket Server:

2. Create a new Multibranch Pipeline project in the classic UI, using a repo on the Bitbucket server you just set up in step 1:

3. Optionally, run the pipeline to make sure it works. The one I'm using here is pretty simple, and does work.

4. Click "Open Blue Ocean" on the left, and navigate to a successful run of the Pipeline:

5. Click the edit button. The fact that the button is greyed out is already written up in JENKINS-46398:

6. Make any legitimate change to an existing stage in the Pipeline, using the editor. In this example I'm changing the length of a sleep from 15 seconds to 1 second:

7. Save the change. Provide a description, and save the change to a new branch

8. Click `Save & run`. The user is shown an error: "Error
Precondition Required." This isn't at all clear.