Lucas Blackstone
Explorer
If the object being used for cover is invisible then the target can be targeted normally because they are plainly visible. The actual material will still provide whatever protection it it can, so an invisible sheet of steel may block an attack but an invisible sheet wouldn't do much against anything except possibly a small thrown rock.
Targeting for spells requires a clear path to the target. It's just how magic works. A target with a sheet between it and the caster no longer meets the clear path requirement. The sheet isn't protecting the target because of how strong of a barrier it it. it's protecting the target because the clear path is obstructed. It's just one of those rules quirks I would expect to be ignored in most games because it is most likely unintended.