Yes it was as I said. On the unit you want the ability you have to research the upgrade.
Also, on the unit, towards the bottom, add 'Backpack (human)' to upgrades used. 'Unit Inventory (human)' shouldn't work without it.
To research the upgrade, go to 'Scenario -> Upgrade Properties' check the box that says use custom upgrades; and scroll down in Human to find backpack (human). Set it to researched and the ability works fine.
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