Sounds like your keyboard has a liquid spill at some point and the traces are corroded. That will need either a new uppercase or if you’ve got the skills replace the keyboard alone.
But like many things in repairing Apples newer systems you’ll need to take the battery out which is glued in! Then getting to the keyboard is a bit of work if you’ve not done one before as its held by a mix of screws and rivets!
Basically taking the everything apart and you may mess it up in the process. Which is why I strongly recommend just replacing the uppercase which also gets you a fresh battery and a lot less work! [guide|27696] and here’s the part: [product|IF117-028] It looks like IFIXIT is out of stock but here’s another source: [https://www.thebookyard.com/advanced_search_result.php?keywords=661-8311|TheBookYard - Uppercase Apple P/N 661-8311] And you can get the correct keyboard for your system (European).