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More layouts and customization options for V3
"Operation is not implemented, or supported, or enabled." is a message that is always caused by V3 itself. Also, it doesn't look like you're on https://www.youtube.com/feed/subscriptions at all, because the subscriptions button would be focused if you were:
It's good everywhere else, however, on the older layouts, I cannot access the Subscribers page, as seen in the screenshot below. Can you fix this?