It is possible to prompt a ‘AllowInventoryReadAccess’ AvatarEditorService:PromptAllowInventoryReadAccess() request through plugins, and the command bar in Roblox Studio Edit Mode.
No UI is shown to the user because it’s likely linked to their LocalPlayer, and CoreGui which is not present.
Expected behavior
Either a prompt should be shown to the user, or it should not be necessary for InventoryReadAccess to be granted by the user when things like AvatarEditorService:GetInventory() is called from the command bar, or a plugin.
Prompting for player inventory access is not currently supported outside of a Roblox player or studio play client context. We don’t have immediate plans to change this, but I have added a ticket on our backlog to improve the error messaging in this case. Thanks again for the report!