Oh, OK. Now I understand the //.
Well, my suggestion is to eliminate the line in your sources.list file (or it might be a file under /etc/apt/sources.list.d) for Ubuntu Precise. I'm guessing that line will look like: deb http://repo.mongodb.org/apt/ubuntu precise mongodb-org, or something similar.
That leaves you with the line for Debian Buster. Find that line and replace any instance of "i386" with "amd64". Save the sources.list file, then do another apt update.
Hopefully that'll get you somewhere.