Apple TV doesn't allow sign in via Safari on an iMacOS High Sierra

Hey there!

From the Apple Support article Watch in a web browser - Apple Support, Apple includes the following note:

On a Windows PC, Chrome OS device, or Android device, you can watch Apple TV+ and MLS Season Pass in the Chrome and Firefox web browsers.

Mac devices and Safari aren't supported.

However, you might be able to upgrade to a newer version of macOS to get the Apple TV app. To see if you can, let us know the year of your iMac. The simplest way to find out is About This Mac, available by choosing About This Mac from the Apple menu  in the upper-left corner of your screen. 

-Jack

iMac12,2 corresponds to an iMac (27-inch, Mid 2011). For that Mac, High Sierra is, indeed, the end of the line.

That rules out upgrading to Catalina or later to use the TV app. Watch on a Mac - Apple Support says that for Mojave or earlier, the other alternative is to use Safari to go to the Web site tv.apple.com .

Watch in a web browser - Apple Support seems to be written specifically for NON-Mac users. It's been a very, very long time since there was a version of Safari that ran on Windows. There were never versions of Safari for Chrome OS and Android. So it makes sense that Apple would not mention Safari in the context of Windows and Chrome OS and Android.