Unable to Enable iCloud Photos Despite Enough Storage and iCloud 2TB (iPhone 12 Pro Max)

"Unable to Enable iCloud Photos Despite Enough Storage and iCloud 2TB (iPhone 12 Pro Max): When I try to turn on iCloud Photos (Settings → Apple ID → iCloud → Photos), it shows “Unable to enable iCloud Photos” / the toggle cannot be turned on.[...]Are there any additional steps I should try before contacting Apple Support? Thank you in advance for any help."

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Troubleshooting iCloud Photos Sync:

So, is the toggle dulled out? If so, it is likely a sync issue.

A. Force Reassert your iPhone:

Go here: Force Restart iPhone - Apple Support

B. Be Logged into the Same Apple ID:

Be certain your are logging into the same Apple Account (email and password)

C. Perform a New Apple Login Session:

Sign Out of and then back into your Apple Account in Settings.

ryno230 wrote …• Restarted the iPhone multiple times

TheLittles suggested you do a "Forced Restart," and you didn't say if you had done that. This is different from a "Restart," a simple power off and on.

Force restart iPhone - Apple Support

I had to continue holding the button down, may be 5 or 10 seconds past the "slide to power off" message, and then I released the button only after the Apple Logo appeared indicating a re-start. 

This is pretty weird. Do you use iCloud Drive on your iPhone? Have you tried an iCloud backup? (I'm curious if all iCloud is compromised, or just iCloud Photos.)

Did this situation just change, or has this iPhone never connected to iCloud Photos?

"Thank you for the suggestions. To clarify, the iCloud Photos toggle is NOT greyed out. The switch is visible and tappable, but when I try to turn it on, it immediately shows “Unable to enable iCloud Photos”and stays off. I am using one iPhone 11 and one iPad Pro, all signed in with the same Apple ID On both the iPhone 11 and iPad Pro, iCloud Photos can be enabled and syncs normally without any issues."

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Troubleshooting iCloud Photos Sync Issues:

A. Delete ALL VPNs:

Are you using any VPNs here? If do, delete them all.

B. Update ALL your Devices:

Back up your mobile Apple devices to a computer. Then, update it to the latest operating system. Backup, once completed.

C. Force Restart your Device:

Perform a Force Restart on this device being your the issue.

D. New iCloud Sessions:

Log out of iCloud on all of your Apple devices, and then log back in.