Add Step Details Error for Flagged Emails

Hi everyone! I have a weird error, and I haven't been able to find anything online with a solution. 

I'm trying to move a flagged email to a list, which I know you can do. But when I flag an email, I get an error message to the "open details" pane. When I open it, the Add Step portion is red, so my assumption is that it needs steps for whatever reason. However, even if you do add steps, it doesn't clear the error, so I can't drag it to the appropriate list. 

Any ideas?