How can I be in the sudo file on MacBook Air
It is giving you that error because: 


your specific username is not configured in the sudoers file
your specific user group is not configured in the sudoers file
you are not a member of a group defined in the sudoers file.


macOS, by default, configures the members of the wheel group to have sudo permissions.  All admins are a member of this group.

If you are not an admin, or you have changed your groups associations and/or reconfigured the /etc/sudoers file, you will need to make the appropriate changes to grant yourself sudo privileges.