OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
MacBook Pro A1286 disassemble to detect memory slot not pluged correct and water damage - YouTube
SOLVED: Issues with bottom RAM-slot MBP 2012 - MacBook Pro 13" Unibody Mid 2012 - iFixit
RAM card is stuck in bottom slot 2012 MacBook Pro : r/applehelp