Unable to sign in iCloud is not currently available. Try again later.
Why does AppleTV (HD) not let me log in to iCloud?
Apple TV 4K (1st generation)
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While it is possible that services might experience unexpected temporary outages, iCloud isn’t currently mentioned on the system status page as unavailable now or recently.
Is this about the A1625, or an earlier model (A1469, A1427, A1378)?
Identify your Apple TV model - Apple Support
Find your Apple TV model number - Apple Support
For those earlier models, those might not be able to adapt to recent changes in (your) iCloud. Please contact Apple Support for their compatibility statement for the combination of iCloud services (Photos?) and your exact model Apple TV.
A1625 would have support for any updates to iCloud services and the like, as its system software can get routinely updated. Update your Apple TV - Apple Support