Why can't I make changes to the lightening actions

Hi,

I was working on a module.  I had done all the steps, but I didn't have replacement owner as an option in lightening. I went into the Case Page layout and it wouldn't let me add it to the lightening actions. I could add it to classic.  I was able to complete the task in classic, but I need it to work in lightening. What would be the reason for this?  Did I miss a permission somewhere?

Here is a screenshot:

#Trailhead Challenges  #Trailhead