Can't sign APK for google play store export,How to use Keystore with unity after update?

I made an app for the google play store about a month ago. Everything was fine, I exported the APK from unity, dropped it in the google play console and poof it all worked. I was shocked, but hey, nice. I updated the app a few times and tested everything out. Worked great. This week I noticed a bug, so I decided to make another update. When I went to drop it into google play console I got an error I'd never seen.

I looked around for a bit and could not find any solutions that worked for me. I made sure debug mode was not checked when I exported. I verified my build codes were right in player settings. Online many posts say the problem is from the publishing section of the player settings. That's where it seems you actually sign your APK. The thing is I never did this before. When I originally published it and updated it I never touched this. The section was blank. Google play provides a list of App signing keys online, but I never used them before either and even so I don't see a spot for them in Unity. There seem to be no tutorials online and everything I try does not work. Why does it want me to do this now and how does one do this?