What device?... The ESPN app is devious. It sometimes purposely hides the SEC+ option and tries to make you go through an ESPN+ subscription.
If you're on Apple make sure the app is updated. Last night I didn't have the option to choose my tv provider until immediately after I updated. Also if you're on Apple.. make sure you go to Settings/(scroll way down)TV Provider and make sure you've got the right one selected. I use YT TV and last night mine was still set to DirecTV so it wouldn't let me log in.
Finally... regardless if you're on Roku, iOS, etc.. make sure you're clicking on the SEC+ broadcast and NOT the ESPN+ broadcast. Sometimes they're side by side.. in other apps you click on what you want to watch and then choose which broadcast.
I'm a minor expert at this because I have multiple 80 year old people in my life who think I must be the love child of Steve Jobs and Bill Gates when it comes to fixing their streaming issues.