Thanks for trying the new experience out. It sounds like you are inside the 'optimise' view because of the striped bars. Drag and drop is not possible in this view because it is a preview of Portfolio's scheduling algorithm.
You can either:
1. "Cancel", and this will return you to the regular view.
2. "Apply changes" from optimise. This will not affect your data in Jira until you review & save changes.
After one of these, you'll be in the regular roadmap view where editing is possible.
Let me know if this answers your questions, or if you have any more feedback on the new experience.