Can't authorise settings change or install new apps
Navigate to Macintosh HD->Users and see what your username folder is, and use that when prompted.

Sometimes the username and name you see aren't always the same. As an example, I have profiles named Christian (Work) and Christian (Personal). The actual usernames are christian and christianpersonal, respectively. When prompted for an admin, I have to use christian to gain access. This is because I have changed the profile name after it was first created, giving it a different viewing name than the user folder.