First, you have two apertures in one aperture pad: Mask and Paste. The “metal mask opening” is the Paste aperture, at least that’s how I interpret it because the paste stencil is metal while the solder mask has nothing to do with metal. You should have only “Paste” ticked in your aperture pads. The copper pad should have Mask ticked but not Paste.
The error message means that there are two different items belonging to two different nets under one mask opening. The mask opening doesn’t of course have any net. I don’t know why this happens here. Try to fix your pads as I explained, maybe that helps.
If not, How does solder mask layer work? may give an answer. Maybe the mask bridges/webs/dams are removed and KiCad v7 handles it as an error? If so, IMO it’s a bug because a large mask opening is expected to bridge items in that case. At least one mask opening should give only one error.