Admin account does not have sudo?
MacBook Pro with Retina display, OS X El Capitan (10.11.5), Soon Running macOS Sierra beta
The default sudoers file has the "admin" group set for using sudo.
You can check the membership of the admin group with Directory Utility.
On the Directory Editor tab, choose Groups from the Viewing Popup menu.
Select the Administrators group.
Look for the GroupMembership line and click the triangle to disclose the contents.
If your user is not in that group, you can try to add it. You must first click the Padlock to authenticate.
If you are already in that group, or you cannot authenticate, you may have a corrupt sudoers file.
You may be able to create a new admin user that can use sudo. If so, you may be able to then repair it from that user.
If you are in the admin group, then you try creating a new admin user.
Log into that user and run this in the terminal
sudo visudo
Opening it and saving it may correct the problem. However, if it doesn't open or filled with garbage characters, then you might need to edit it.
If that is the case, I can show you what it should look like.
You can learn to edit in Vim by running this command in the Terminal:
vimtutor
Admin account does not have sudo?