When you have sufficiently large data, or a large number of directives, or longer column names, add to pipeline doesn't work.
It simply shows the "User Interface Service is not available" message.
The bad part about this is - after this happens, even if you re-upload the data, and you add only 1 directive, add to pipeline still doesn't work.
I will attach a sample dataset to reproduce this issue. To reproduce, run data preparation on this dataset, by adding multiple parse-as-json and flatten directives on columns that look like JSON.
I have a suspicion that this is related to
CDAP-8844, however, we should investigate why add to pipeline becomes unusable, even after you've discarded all directives.