How can I enable "Create Extension" menu item as it is currently disabled

The "Create Extension" menu item is disabled. How can I enable it?

I'm aware of the fact that the model that owns the table i'm trying to extend must be referenced.

However, after following Dynamics 365 > Model Management > Update Model Parameters. The model that owns the table I'm trying to end isn't available for selection.

What could have gone wrong?

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