Getting “Whoa! There was an error logging in with that account.” On a latest gen AppleTV. Can log in on other devices.
Welcome to the forum Bradley.
As annoying as this is, try uninstalling and reinstalling the Zwift app.
Failing that, perform a full reset of the AppleTV, followed by updating the OS, reinstalling the Zwift app. If you have an iCloud account and other iOS devices, it should go quickly.
Also of note: force close the Zwift after after each Zwift session. While it shouldn’t be required, it largely is on ATV to prevent glitches.
I saw that exact message after upgrading to a new iPad (even though Zwift showed my account as active on the main screen) - to resolve it, I used the option to switch user and entered my account info again.
This seemed to goose things back into order without resorting to other workflows.
Worth a try before going deeper into uninstall/reinstall or full ATV resets.
CJ also makes a great point about manually closing out Zwift on AppleTV - this does go a long way in my experience toward ensuring a stable experience on that platform. In case you don’t already know how: while on the main Zwift screen, double-click the TV/Control Center button on the remote, then swipe up on the touch pad. This ensures that Zwift isn’t “parked” in the AppleTV’s memory and allows each launch of Zwift to be truly fresh.