If you create a workflow with a few steps, but don't add any transitions to them, no transitions can be added and the steps cannot be deleted once the workflow scheme is associated to a project, even if you make a draft workflow.
As soon as you disassociate all projects from the workflow scheme, you can add transitions or delete the step.
How to reproduce:
1. Create a workflow.
2. Create multiple steps in the workflow but add no transitions.
3. Create a workflow scheme.
4. Associate the workflow to an issue type in the workflow scheme.
5. Associate the workflow scheme to a project.
6. Create a draft of the workflow.
7. Try to add a transition to any of the steps.
8. Try to delete any of the steps.
This is not a support request, as I have a workaround: make sure every step has at least one transition before associating them to a project.