Sender Details in Automations-Can't Press Continue

I’ve sent one test email. Now trying to set up an automation. I’ve selected a trigger and I’m trying to add a campaign. I select any email or template and when I do sender details pops up. Right after I enter a subject line, it won’t let me continue. What is the most common reason for that? Something I probably haven’t set up?

Hi Katie. Are you referring to the Sender Details modal (pop-up)? If so, can you post a screen shot of that pop-up?