While rerun a process step in Admin UI, it shows "This operation cannot be completed without the selection of one or more operations from the list below!"

There was a process running on LiveCycle ES2 SP2 server.

It kept on "RUNNING" status on a step.

Therefore I choose the step and click the "retry" button in Admin UI Process Management. 

However, it failed and displayed such an error:

Error Message

This operation cannot be completed without the selection of one or more operations from the list below!"

Is it a bug? Is there any way for me to rerun the step?

Thanks.