Cant seem to get Commenter Permissions to work

I've a 3 user account which I've signed up to even though its mainly just me who is an active user... what I do want is the ability to share a plan with a client and allow them to add comments to. This all seems like something out of the box that should work...

However, when I add a free user and share the sheet as a 'commentator' they log in and cant add any comments... Can someone please help me understand why this is the case?

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