Beagle Bone Black - Error when finding update packages: sudo apt-get update
There are a few possiblities here.

The most likely explanation: You haven't connected to the internet. If you have an ethernet cable plugged in (on both ends!), you should be able to run dhcpcd eth0 to bring the interface up.

If you do, try running ping 8.8.8.8, to check for connectivity. If that returns something along the lines of 64 bytes from 8.8.8.8, then it must be a DNS problem.

You can resolve that by adding nameserver 8.8.8.8 to /etc/resolv.conf