Constant vertical lines on an LCD panel when you tap it lightly on a table mean a defective LCD controller chip. The problem may disappear, but it will reappear with increasing frequency until the lines are always there. You have three options: RMA; buy a new screen and install it; buy a used screen or inoperative unit with the same screen (could be a different make and model) and swap screens.
If the lines flicker when tapping the tablet on a table, the problem may be a loose ribbon cable or connector, but this is rare, while bad controller chips in cheap panels is common.