Asking for username and password all the time

After doing the new update for Mac it keeps requesting me to enter a username and a password for wanting to change my system preferences, helping with fb messenger and so on. What username and what password is my question? I have tried my Apple ID, other usernames, passwords etc, however it keeps telling me that it is wrong.

Do you have any suggestions what username and password I need to use to enter my different settings?

Many thanks, Emma :))