Fixing the black line on the phone

@ragalugard that will depend. It is possible that this is a bad LCD or that this is related to a bad flex cable, a bad connector or a bad video circuitry. I would suggest you

a. post an image of what your screen shows.

b. consider replacing the display assembly and see if that fixes it.

If it does not you know that the issue will be on the circuit board and will require some good tools and skills to fix that. Oh yes, you will also need the schematic and component layout for your circuit board.

To work on your phone, you can use this guide Samsung Galaxy A12 Disassembly - iFixit Repair Guide or you could refer to a video like this one Replacement LCD's are readily available. Those come with and without the midframe. I would suggest that you get one with the frame already installed. This will make the task a lot easier.

Let us know what happens when you replace the screen.

Adding images to an existing question - iFixit Repair Guide