Can someone explain steam guard to me?

I am trying to sell stuff. It says it's on hold... because I don't have steam guard.

But I do have steam guard, I have steam guard email which has been on my account for like forever. When I click "why are these items on hold" it says because I don't have steam guard mobile authenticator. Then when I click "more info" it says:

"We require Steam Guard (via email or mobile authenticator) to be enabled for 15 days to help protect your items and ....."

So wtf is going on. I don't really understand...

I think it has something to do with me being on a "newly recognized device", but their explanation of that makes 0 sense at all, since it says my device should be confirmed by email but the email that my guard uses is the same email (obviously) that I have to go into to confirm each large sell in the first place.