I have noticed that posters are often asked to run commands in Terminal using the sudo command. If the user is logged in from a regular (non-admin) account as per recommended best practice for security reasons, trying to use sudo will fail. The correct command sequence for a regular user would be su <name of admin user> -c '<command provided goes here>'. At the password prompt enter the password of the target admin user.
Alternatively, login as admin and run sudo <command provided>.
P.S. Forgive me for posting, but this is something I've struggled to understand myself, and I felt that others might benefit if this were a sticky perhaps or at least presented to appropriate posters.