Hi DMDM
Multi-Inventory is not enabled globally by default unless you specifically go to ATUM Settings >> Multi-Inventory and enable that option.
Is there an easy way to activate Multi-Inventory for all variations of a Variable product using WP Admin?
If you want to do this in bulk for all the variation products in your store, perhaps you can achieve this easily with the ATUM Export & Import PRO addon.
But if you want to do so only on a few handpicked products, there is no toggle to set them at once within the admin UI, and you must set this manually one by one.
Best Regards,
Salva