I was having the same issue with safari on my MBP, running the latest macOS with safari, but not chrome. Would say something like “cannot verify account”. Also worked fine on my iPhone mobile safari. Just wouldn’t let me login on my Mac.
This lasted a few days and then fixed itself without me doing anything. I assumed it was just typical brain dead delta IT.

I’ve intermittently also had a problem (also only on safari on the Mac) where I could see all my upcoming flights but not click into the details for any of them in My Trips. I’d have to click on Find My Trip and manually enter my info, even though the flight was listed above on that page. This also only about a week and then started working again.