This is a problem with the inherent design of these boards. One RAM slot is attached via through-hole soldering on the logic board, and rarely fails. The other is attached through BGA, and is very easy to break if the slot is bent or computer dropped or one of Jupiter's moons goes on the far side of the planet. It's something that needs care when working on the board, and it sounds like you may have one bad slot. I've yet to see both go bad at the same time.
Since you said you have 16 GB of RAM, you will have an 8 GB card in each slot. I would start out by removing both RAM cards and only inserting one in the top/closest slot. Run the computer with 8GB and see what happens. If you get no errors, it is likely the BGA soldering on the further RAM slot. If that doesn't fix it, try just one RAM card in the bottom/furthest slot. It may be the top slow somehow failed in a strange twist of fate, that is what we're here to find out.
An actual RAM slot replacement is not something easy or at all straightforward, and not without risks. Many people choose to just use one RAM slot if the have only one good slot. If none of this fixes it, it may need a new memory controller or parts of the memory circuitry replaced.
Anyways, try either RAM slot on its own, see what happens, and we can go from there.
Hope this helps.