Seems like some packages are not present in http://sfo1.mirrors.digitalocean.com/ubuntu/dists. Here is the output when I run sudo apt-get update:
Ign:1 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.2 InRelea...
So I finally figured it out. I have installed resilio-sync which add armhf as foreign architecture (your architecture maybe i386 or amd64). Launch this command to verify: